Something to consider is are any of the quads running extended swingarms? That will make a little difference with fitment.
I have a 7X14 enclosed and I can fit three quads and some gear...just to to you an idea. You will need a pretty good sized trailer to fit 6 quads and a rzr.
I've owned both and for my riding style I prefer a banshee over a 450. Its all going to come down to personal preference though. The banshee will be alot cheaper to make faster, and cheaper to rebuild when the time comes. Stuff to think about.
Few things I would double check:
Carb sync
clogged pilot jet
air leaks
I had an issue similar to that and the pilot jet on my left carb had a tiny piece of sand in it. When you pull you pilot jet you should be able to see light through it. Just a suggestion.
Whats in these things? Any problems guys have had running this stuff? I don't really like mixing anything with my fuel unless its some good old Klotz R-50.