well today i got a betta and put him in a 1.5 gallon. then my sister got one, so i put mine in my 10 gallon along with 6 harlequin rasboras(schoolers, less than 1"), 3 dime-nickel sized snails and 2 african dwarf frogs.
One is in a 1.5 gallon hex with half moss half gravel on the bottom, heater, under gravel filter(dnt see how it works rly bsides water circulation), hood w/ light and cover, and a fake lillyish plant.
One is in a 10 gallon heat,light,hood,filter,moss+sand+gravel+rocks, hanging fake plants which make a good place to hide combined with the aquascaping.
The betta in the 10 gallon doesnt seem nervous at all, hes very interested in everything. He flared when he first saw the other fish and whacked into one once, but after hes established that they arent out to fight hes fine and leaves them alone. He doesnt understand the snail, hell get right up close to it and just look at it, then swim away a minute later. He's nipped the frogs foot once, but the frog kicked him in the face and swam away quickly. since then hes left the frog alone. He swims all over the tank, checking everything out.
The fish in the 1.5 gallon seems good, flares on occasion, and is swimming even though theres not too much room.
I'll update this little expiriment, even though I do think it's just a matter of preference. However, the people with 1 gallon bowls cant argue that legitimately unless they have had their bettas in a 10 gallon as well, the same betta they have in their 1 gallon.