I have a 4 seater, the difference in speed is not much at all. the wheel base has pros and cons.
pro: it rides 100 times better than a 2 seater in whoops and is in my opinion and experience faster in the whoops. it also tracks very well in the tight twisty trails and is very fast.
cons: like green said the WB will get high centered easier but for most stuff other than rock crawling has never been a problem at all. Wont be a problem at all in the dunes.
the weight different of the actual unloaded machines is only a few hundred pounds so again not a huge deal.
what ever you get get a 900 XP
I got paddles for mine at the dunes and that made a big difference.