Yep, I'd definitely recommend the tanks. I too have two of the 3's. I've never had any problem with the filter current, and if it is too much for your betta, just put a tall plant in front of it, usually works. Either the 3 or the 6 would be great, as previously stated. You would have to get a small heater. I have 25 watts and just keep it on a low temp, and it stays pretty stable around 79 when I use it. The lighting is fine for bettas.
Just some things you should know, the light takes a couple seconds to turn on, but that's not a big deal at all, the acrylic is easy to scratch but I haven't had too much trouble with that. The one thing that does bother me, is that there is no small opening for feeding, so I have to lift up the whole lid a little for feeding time. They're all minor things and not an inconvenience really, but just so you know.
Here's a pic of one of mine.
Good luck. :)
I gave an eclipse 3 to my cousin with a betta for his birthday and he loved it.