'05 vs. '06 -- the difference.

Here's the difference in rear tire clearance between the '05 and '06 Dos Niner frames. For the 2.2 Kenda Karma shown in the pics, clearance isn't really an issue, but there's no way a 2.3 WTB Exiwolf will fit in the back of the '05. Since the '06 frame has an extra 15mm of clearance on the top and upper-sides, fitting the Exi in the rear will be no problem at all.

Otherwise, both bikes ride and handle pretty much identically (which is very good).