I keep flip flopping on whether or not I want to divide my 10 gallon tank. The most pressing issue is the silicone I'd use to secure it with, (Home made divider. :}) but I've pretty much get enough in my kitty bank to pay for it. (Its a bank...in the shape of a cat..Yeah. )
Depends. Is there already a betta that's in the whole 10 gallon? Jango has a 10 gal. to himself because I could never bring myself to allow him to have all that room, and that divide it and let another betta into his palace. But my newer tank will be divided, as it will be an upgrade from what Takeshi and Chai are in now. I just don't like the thought of downgrading the amount of space they have.
I'm torn on the idea. 5g is plenty of room for a happy betta home (and having a divided 10g means you can easily keep a stable environment for both fishies), but seeing a beautiful betta cruising a 10g that's his or her own is great sight, too.
If you do divide, make certain you have something on both sides of the divider that will agitate the water surface (like a small bubble stone).
I didn't vote, but wanted to say I was torn for awhile too over whether to keep one in my 10g alone, or divide it. I ended up dividing it, and decorated each side enough that there's plenty to keep both entertained. :) I used the topfin divider from petsmart and all has been well.