I wouldn't buy either of them.
2.5 gallons is great, but those filters you can't adjust, so it may be too strong, and i hate the grating on the filters, it just screams betta fin death trap-- meaning if it's too strong, their fins can get sucked into it if they swim by, possibly tearing them, or getting Boo stuck to it.
Bettas aren't really a dirty fish, so a filter isn't really needed, plus they like no-current water, and filters create currents. the smaller the tank, the greater the current.
I suppose if you could take the filter out, then yeah, the 2 and 2.5 gallon would be good. But you still need a heater, don't forget.
personally, i just get the 2-5 gallon keepers or tradition aquariums. you can get 2.5 gallon glass/acrylic ones too.
but what I usually go for, for a single betta, is like this:
Medium to large size keeper - $10-$13: http://www.petco.com/product/12031/P...t=OnSiteSearch
A heater for the size of the tank (2-10 gallon tanks) - $15 : http://www.petco.com/product/114078/...t=OnSiteSearch
And a light, though optional, it looks nice, it be anything you already have but don't use, like an old desk lamp, but they sell lights for keepers online that you can by too. http://www.amazon.com/Lee-Kritter-Light/dp/B0002APZNA
it says they fit Lee brand keepers, but i imagine it might fit petco brand too, if not just sit in the hole instead. -shrug-
then you just get all your decor, which can be anything you like, everyone has their own preference. gravel, marbles, "flattened marbles", sand; for subsrtate, real or fake plants, and then either fabric or plastic fake plants, then caves; plastic or ceramic, they even make glass ones. but you can use anything for caves. I've used old coffee cups, terra cotta pots, mouse/hamster tunnels. be creative. You can even build something out of legos, which i created a stir about a while ago.