We had guinea pigs when I was growing up; we had the long-haired guinea pigs, who were so cute! Now my girls have hamsters, and I like them MUCH BETTER! Don't get me wrong -- the piggies were loads of fun, but the hamsters are better.
They're practical: Being smaller, the hamsters fit into a smaller cage. They make smaller messes and the cage doesn't need to be cleaned as often. They eat less, and they use less litter. Of course, all this works out to less money out of your pocket.
Both critters are equally cuddly and sociable (with humans), but the hamster can also run around the house in an exercise ball, which is entertaining for us. We've never lost our hamster, but that might be a concern with a small child.
Guinea pigs like animal company, so you want two of them.
Dwarf hamsters like animal company, so you want two of them -- they will not become cuddly-friends for you.
Syrian hamsters (sold as "bears" -- Teddy Bears, Panda Bears, Honey Bears, etc.) are much larger than the dwarves, and they do not want to share their cages; they're very territorial and will fight violently. They will, however, become very affectionate to their human owners.
I don't remember ever washing the guinea pigs.
Hamsters cannot be bathed; they do not have the ability to regulate their body temperature, and they can literally die from a bath. Our hamster care book says that if they get into something (i.e., oil), they should go to the vet, who knows how to handle them when wet. I don't know for 100% certain that this is true, but I also am not willing to test it on a critter whom my daughters love so much.
As for them living outside, I used to take my guinea pigs outside -- they lived in a wire cage -- and they LOVED to nibble on the fresh grass that'd poke through their cage. They did not live outdoors all the time.
Plus it will feel extra good to give a couple of little guys a nice safe home.
