Some people say nothing under a 5G can hold a stable cycle and others claim they have cycled 1 gallons.
If you have the right type of lighting, it may be worth using live plants. personally I dont bother with trying to cycle anything under 5G- simple reason is I do not have enough outlets for heaters AND filters.
If it were me and you have the space/money - I would go with the 5G. Nothing wrong with a 2.5G, it's just my personal preference. More swimming space for them, more room to decorate for me :D Even better - a 10G. It can be divided to hold 2 or 3 bettas or you can add some bottom feeders like cory's.