My recommendation to you is a Can-Am ATV 500cc or more and the Can-Am Apache track system
Apache Track System - Can-Am - ATV.
Can-Am will give you the straight up power, the best power-to-weight ratio, and the most advanced features of any ATV. Note that you will pay more for this brand than any other, but you get what you pay for. A Can-Am 500 should have no problem keeping up with most 600-650cc machines. Also all 2010 machines 500cc+ from Can-Am w/ the XT package include front and rear bumpers, winch, aluminum rims, best stock tire you can get (Carlisle ACT), handguards and 2 mode dynamic power steering, which is IMHO the best power steering system on ATVs, and I have tried them all. The Can-Am + Apache track setup is by far the best way to go.
If you don't have the money to invest in a setup like that, the second best route would be either an Arctic Cat 550 H1 EFI or 700 H1 EFI, or a Yamaha Grizzly 550 EFI or 700 EFI. Along with either an AC or Yamaha I would get the Litefoot ATV track system.
LiteFoot ATV Rubber Track Conversion Systems which seems to also be a very good track setup.
In my many years of riding I have seen the most breakdowns and general problems in Polaris ATVs. My in-laws own them, and buy 2 new ones every year, and some of my friends and other relatives own them, they are the only ones that every break down on the trail, or run into problems elsewhere. The rest of the pack includes Arctic Cats, Can-Ams and Yamahas, all of which have been trouble free on and off the trails, and a Kawasaki and Honda which both have run into problems.
That is my recommendation from many years of riding and experience on all makes and models of ATV. Hope it helps.
By the way, the rule of thumb with track systems seems to be that you will gain a wider stance and 4 to 6 inches of ground clearance and lose roughly 10% power and 30% top speed.