Tanks because thy generally tend to have more volume, more space for the fish to swim in, and much easier to fit equipment like filters/heaters/lights in if you ever decide to use those things.
Crap... maybe I should of done that with my last betta T_T!!! So much went wrong with that one T_T!!! This one doesn't seem to mind it though... I don't know if he likes the bubbles or not from it. He's only been in thre for about 4 or so days so far.KitKat88, yes filters are good for fish, but bettas are a bit of an exception. They produce such little waste they don't necessarily need a filter, and they hate current in the water. I have a filter in my bowl (at a very low flow) but some bettas don't like filters even on a low flow. It kinda depends on the individual fish. I would always try with a filter first and see whether the betta is OK with it. If not, I'd remove it.