+1 for kenda klaws. They really are one of the best tires out there still, and they're as cheap as they get too. They have reversible tread, so you can run them one direction for maximum traction in loose stuff, then flip them if you'll be running on hard pack and you'll hook up like crazy. They last forever too, in most cases they can outlast just about every other part on the quad.
The thing about razr mx's, really any mx tire, is that they're only 2 ply. That's usually ok for the mx track, but take an 18" 2 ply tire out in the trails for a day and tell me what condition that tire is in when it's all over. Bet it's not quite so full of air as it started.