Rats and mice need a certain amount of protein and fat in their diet; you shouldn't feed them "low-fat" foods, as they are omnivores -- unlike hamsters, gerbils, and guinea pigs, which are herbivores and don't need as much fat.
Of the Rodent family, rats make probably the best pets. They are clean (as long as you clean their cages regularly; you can even litter train them, if you know how), and they are more friendly and intelligent than any other pet rodent. The only thing people don't tend to like is the tail; it grosses them out, I guess, but you get used to it. Rats also live longer than most small rodents, on average.