I have an A-bolt in 300 win mag and absolutely love it. It shoots great and has never let me down. I bought my son a X-bolt in 300 win mag a couple of years ago and it is a great gun as well. It shoots a little better than my a-bolt and I really like the feel of the x-bolt. I also like the detachable mag on the x-bolt.
If I was to buy one or the other tomorrow, I would buy the X-bolt.