Hey there PaulO :)
Well that certainly is lucky, finding out you can fit a bigger tank on your stand! Personally if I had the option, I'd go for a 20 over a 10, especially when we're talking about a Sorority; more space is always better. As for taking back the 10 gallon.....thats all up to you. If you have the time/extra space/funds to set up both a 10 and a 20(or if you think you'll want to set up the 10 later) then you could keep the 10 and go snag a 20....though if you're just looking to deal with one new tank, you can take the 10 back and get a 20.....all up to you really.
Because I'm a crazy fish lady, I'd probably keep the 10, go get a 20, and set up the 10 later/gradually put it together when I had the funds. But thats just me xD
Here is our sticky about Sororities http://www.bettafish.com/showthread.php?t=29402
if you haven't already read that over. Besides the original post, there are several pages of people asking questions and giving advice about setting up Sororities, and you can also search the forum for posts other users had made. Theres TONS of fantastic Sorority information on this forum alone, and lots of experienced keepers willing to answer any specific questions you might have about setting one up. ;)
Read all you can! Theres no such thing as too much research~
As far as filters go, I'm partial to your standard HOB(Hang on Back)filter(Aqueon is a favorite brand of mine, though I have a Top Fin in my 10 that I'm liking quite a bit as well)with a baffle. There are a couple ways you can go about baffling your filter to help slow the water flow for your bettas; you can cut out the toe of a pair of pantyhose and secure it over the outflow with a rubber band, or get some filter sponge and secure it the same way over the outflow. You can also secure a some pantyhose over the intake of the filter so no one gets sucked into it(I've had troubles with that in the past, so I do that with all my tanks now)and I find that helps slow the outflow a little as well.
Aqueon filters tend to have a stronger outflow IME, but I've had luck with a filter sponge+pantyhose combo over the outflow.
I hope that helped a little and answered some questions, feel free to ask anything else that comes to mind. :) Best of luck!