They're vaguely similar. Prairie dogs are slightly smaller I believe but I'm not sure that they would make very good pets.
I see them being sold in stores pretty often but I wonder how they would do once they reached sexual maturity (a good amount of "wild" animals made pets--raccoons, squirrels, etc.--can get aggressive once they hit that age). My cousin had 2 of them but they were very young. I think she got them neutered/spayed though. Maybe that would help counter possible aggression issues. She wound up getting rid of them anyway. I don't know why.
But I would make sure you have habitats that are very separate if you're going to have a prairie dog. I think they eat a decent amount of animal protein and probably wouldn't be against taking a bit or two out of the leg of a tortoise.
I have heard they make really good pets its just the males go into rut for about a month out of the year and can get aggressive during it.