If you are going to get a hamster, Syrians are better because they are bigger. But a small fragile animal and a 6 year old child isn't a match made in heaven. Hamster can bite and when they do, it's to the BONE! Imagine being stapled by a staple gun. Plus they are nocturnal. And as other posters have said, can be very noisy at night. You can get tank style cages to prevent bar biting (only someone who has been kept awake by this particular torture will understand), and a silent(isn) wheel.
Syrian hamsters need big cages approx 50cm x 80cm (mine is 100 x 50). They must be kept alone, as they are very territorial and will fight to the death. If you do get one, get it from a proper breeder, as this will be handtame and healthy and will have been socialised and chances are, will not bite unless you do something stupid or disrespectful. To find a good breeder check out HamsterCentral and post a query to find breeders in your area. If you are in the London area I can recommend a breeder.
But, they are not ideal for little kids. They can die easily and even if you are lucky will live to 2 if you are lucky. Plus they are fragile little creatures. Vets bills are no less because the animal is small. (As I can testify.)
Buying from petshops, both independent and the chains, risks you getting one which has been bred in a rodent mill where the animals are kept in pretty atrocious conditions. Which you may not mind. But you will care about the animal possibly not being as healthy as a 'breeder' hamster (here comes those vet bills), or as importantly, more likely to bite.
Before anyone bites off my head, I will say that these are generalisations. Many people have healthy sociable hamsters from petshops. I was very unlucky, and now have a breeder-bred hamster, 'who' is a delight.
GOldfish. Don't bite. Don't make noises. And when your child loses interest (a very likely scenario) maintenance is pretty low.