Also depends on how active you want it. Betta's generally are pretty lazy. Guppies, from what I've seen at the LFS, are really slow swimmers and just look pretty. DG's are actually really active and swim like a maniac half the time.
Activity isn't that important really, just need them them to be tough little fellers. Also what should the pH be for freshwater?
That's when personality comes into play too. I just put my betta in a 20G long, and he just lays there on the plants. My DG in my 55G either swim along the top, chill in the plants or chase each otherBettas are not lazy. They can't swim fast like some, but I've never had a lazy betta. They get lazy in those tiny cups and .5 gallon bowls they get put in sometimes, but not in a decent sized tank for them. Guppies aren't really slow either. They are pretty, but they're fun to watch too. If anything, DG's get lazy. Mine would rather sit in a plant most of the day than frolic and zip around, unless I give him food.