I'll mostly echo what Clark said, but would double check the air filter on the RZR as well. I've seen lots of top end work on Polaris in the local shop, so they aren't immune from dust either.
Private sale or dealership? Warrantied by seller? Which one fits you better?
And of course, price.
What kind of riding are you expecting?
The RZR has a history of catching fire and there are LOTS of recalls for this so make sure the machine has been kept up to date.
Drive both. The drive trains are quite different. The Z may be louder (drivetrain noise) so you will want to check that. The Z has much better engine braking which you will appreciate if you ride a lot of steep hills. The Z also has a locking front diff which can make a big difference when climbing over obstacles, especially uphill.
Check out the RZR forums too. I can't agree that RZRs are more reliable. I've seen too many break down on group trips in a whole host of interesting ways, but then, the owners were spirited drivers, so it's to be expected. I've had the most fun jumping in a RZR, so they have good suspensions.
Whatever you by, expect regular maintenance activies to keep them healthy.