What are the Differences? 

As I've got so many different types of rodents, I thought it would be useful to have a list of the different requirements each animal has and what they are like to keep.

I posted the following to the alt.pets.rodents newsgroup in response to a question.

Rats: 
Very social and interactive. They need to be kept in single-sex pairs or groups .Quite smelly though! If they are handled from a young age, they shouldn't bite. If you've been handling food and they smell that on your fingers, they may bite, but that's because their eyesight isn't too good and they rely on smell to tell the difference. Even if
they haven't been handled, because they're a very intelligent pet, once they get used to your smell and the fact you haven't eaten them, they are great. They can be hand-fed and even be trained to do tricks!

They are active most of the day, having rest periods, then periods of
activity.

They make no noises, apart from the odd squeak of protest if they don't want to be handled. Most other communication is ultrasonic.

However, they can have health problems, thanks to Mycoplasmosis. In itself, it doesn't cause the animals any harm, but it can cause respiratory problems and if those aren't treated, death. :-( Regular visits to the vet tend to be a feature of rat ownership. :-(
They also can't tolerate too much protein in their diet (it causes skin problems). The problem is they'll eat anything, so to be sure, stick mainly to a proper Rat mix for feeding. 

If you can put up with the regular cleaning out, they are VERY rewarding pets. Out of all our rodents, they're the ones I like best.

Hamsters:
Syrian hamsters tend to be less sociable and can bite. Dwarf hamsters are more sociable, but I have trouble handling the things because they slip through my fingers! I've not been bitten by our dwarf ones, but have been bitten by a Syrian in the past. Syrian hamsters are really a nocturnal animal, so when you want to play with them, they want to be asleep. No wonder have have a nip in protest!

Dwarf hamsters tend to be active during the day, but have rest periods as well.

No real health problems.

Quite smelly.

Syrians have to be kept alone. The dwarf varieties can be kept in single-sex pairs, but check first as some have special requirements.
They don't make any noises as I believe all their communication is ultrasonic.
We have a pair of Roberovskis, which are very, very small (2inches long, max) cute looking. With their white eyebrows and muzzle, they look like a pair of old men!

Guinea Pigs:
Very sociable animals and usually don't bite. When handled, tend to just sit there in your lap and can be stroked and hand-fed.
Can be noisy, making all sorts of lovable noises to each other. Usually quiet, but make noises when alarmed, when greeting each other or when they just want to make a noise. Not as bad as a dog barking though!

Should be kept in single-sex pairs. (Can't stress this enough... they're as bad as Rabbits!)

Pretty smelly and need cleaning out regularly.

For some strange reason, certain people exhibit strong allergic reactions to Guinea Pigs.

Degus:
In size, half-way between a hamster and a chinchilla. Not recommended for kids to keep as they need a strict diet. No sugar (e.g. fruit) otherwise they contract diabetes, not too much fat (e.g. nuts and fatty seeds) or they get liver disease. 

A bit vocal, making regular loud "weep!" noises.

Also a bit smelly and need cleaning out at least once a week.

Tend to be the same to handle as a Hamster or Gerbil. Don't tend to bite, but have this thing of the "welcome nip", where they check you out by a little nibble. Can startle people that don't expect it!
They tend to gnaw excessively, so cages should be all-metal, including the base. 
They need a dust bath which can go everywhere, so can be messy animals.

Chinchillas:
Cute bundles of fur. They're nocturnal animals, so sleep during the day and come awake when you're going to bed. 

Don't put their cage in a bedroom! They do make vocal noises, but generally quietly: the problem is noise from the cage while they bounce around from shelf to shelf.  Need a big cage though and even that isn't enough, they need exercise. So be prepared to let them run around your room, with all the preparations that requires (no loose cables: they'll get chewed). 

Not smelly, but can be messy, thanks to their need for a dust bath and the fine fur
gets everywhere.... Because of this, they should be cleaned out regularly.
They do need to have something to gnaw on constantly. 
They have dietary requirements different from most rodents. They need a constant supply of hay and chinchilla pellets adding to any feed they have.
They can trigger allergic reactions in some people.

Gerbils:
Best of the bunch. Quiet, although in a really quiet house, I have heard them quietly squeak (twice, in over a year of ownership!). They do foot-drum occasionally as a form of communication, which sounds like they're scratching and hitting the side of the cage/tank with their feet. Nothing excessive: I might hear it once a month!

Very clean pets, especially in a gerbilarium. Can be handled and don't bite. They are quick
and can jump, so need to be handled with a bit of sympathy. 

Should be kept in single-sex pairs.

Watching them work away at their tunnel complex is quite fascinating, whether it's digging, shifting dirt, or building nests. They tend to re-model the tunnels on a regular basis, so the layout is always changing. They do like burying things, including their food bowl, or anything else you put in. They love gnawing at things, so they will appreciate your used loo-roll collection!
My second-favourite rodent after the rats and the one I always recommend to people wanting a low-odour, low maintenance pet.

Jirds:
Although closely related to Gerbils, they are different to keep as pets. 

First thing, they are larger and require a lot more space to move about in. They need a good size tank, like 4ft. They also like to climb, so the tank will need some sort of cage fitted on top. Unfortunately, I've not seen anything available like this commercially, so you'll have to build your own cage. Ours is a tank with a rat cage on top.

Second thing, depending on which species of Jird you get, they have different temperaments. Females can be aggressive and some shops refuse to sell them in pairs. However, related female pairs should live together fine, as long as they were introduced at a young age and there's enough space to get out of each other's way. There may be the odd scrap, but no fights to the death! Males seem to get along fine in groups.

They aren't as clean as Gerbils, ours do smell a bit. They like to dig and scrabble about, so a tank is best to prevent mess. If you fit a cage on to of the tank with shelves, then the shelves need some sort of guard round them to stop food and poo being thrown out of the cage and onto the table or floor. Considering they're related to Gerbils, most experts advise against housing them in a Gerbilariium.

Like all rodents, Jirds love to chew. But they take it to new extremes: they're the only rodent we've got that has successfully chewed through a 2mm thick hard plastic waste pipe. A supply of small blocks of wood for chewing is useful. Buying long lengths of wood from the local DIY store and cutting them up should be cheaper than buying chew blocks from the pet store. Make sure the wood is untreated. Luckily I've got plenty of offcuts left over from DIY projects.

General comments:
Hamsters and Guinea pigs do smell quite a bit and need regular
cleaning out.

If smell is your main concern, then why not consider a Gerbil? They're the least smelling small pet as far as I'm concerned. Because they're essentially a desert animal, they urinate less than other small animals.

Especially if you make a Gerbilarium for them. They can go without being cleaned out for a couple of months. So in that time you should be able to convince your son to clean them out! ;-) Even going that long, the only smell is the smell of the peat.

I find I have to clean out the animals to avoid smells in the following order of smell:

Guinea Pigs (indoors): every other day. 

Rats: Every two days.

Jirds, Degus and Hamsters: Once a week

Rabbits & Guinea Pigs (outdoors): once a week.

Gerbils: Once every two months.

Cages:

It's best to avoid cages with mesh shelves or floors, as animals can get a problem called bumblefoot. Manufacturers do supply cages with mesh shelves, but you can put wood over them.
The wood also gives them something to gnaw on. Rodents front teeth constantly grow, so they need a supply of things to chew on, like toilet roll tubes, chunks of wood, or like our rats, even small dog chews!
Cages with plastic bases will be chewed, as will any plastic items you put in their cages like houses or tunnels. Wooden or cardboard items tend to be better as plastic when chewed can have sharp edges.

Buying:

One recommendation I would make is if you can, to buy the animals from sources where they handle the animals regularly, so they're already fairly hand-tame. 
Pet shops and bigger breeders may not have the time to handle all their animals, so you end up with fairly wild animals and have to start taming them from scratch. Something most kids don't have the patience for....

As always, check who you're buying the animals from, first off how good are the conditions they're being held in, then see how the animals behave when they're being handled, etc.

Breeding:

Most small rodents can breed faster than Rabbits, so beware. Wherever you buy from, make sure the males and females are separated to avoid bringing home any unwanted broods. There's no guarantee that animals kept in these conditions are unrelated, in fact it's more probable that related animals have mated, so there's an increased risk of birth defects in the new offspring.

Remember, breeding age can be as young as 5 weeks in some species.

If you decide to breed animals deliberately, then make sure you have somewhere for the new offspring to go once they are weaned.

Aftercare:

Always make sure you're able to look after whatever animal you buy. That means proper food, bedding and housing. Not to mention putting something aside for any unexpected vets bills. Especially if you keep Rats.