If you're planning to do a lot of joint stuff/making a lot of collaboration parts, then I'd suggest Flash 8. The majority of the community uses this version, so you'll be able to send file ans whatnot a lot quicker to each other!
Sadly, Flash 8 isn't available on the official site, and the company that created it no longer exists, so I prefer you buy it on Amazon or eBay. Just don't pirate it. :)