I have an x-bolt and an a-bolt, IMO, the x-bolt is definately superior.
-the magazine is a rotary design that is very reliable and tough. The magazine system on the a-bolt is a pita the way that you have to open the floor plate to insert it. On the x-bolt it just pops in.
-the x-bolt has an innovative bolt that allows it to be openned when the safey is still on, thus reducing the chance of AD.
-the scope mounting bolts are greatly improved by the fact that they increased them from 2 to 4 in the front and back.
-the x-bolt has a built in limb saver.
There are other nice features as well like the rubberized stock, however I would say that the magazine system alone makes it worth going with the x-bolt.