My choice is obviously the Xray. I have the 2011 and love the car. It's very easy to drive and super stable on the track, hands down the best buggy I've ever driven.
The Mugen is also a great car, however it is a bit weaker than the Xray. Mugen has a strong tendency to break arms fairly easily and I see it happen all the time. I can't say first hand, but I'm hearing from others that the Mugen lacks a little bit on steering when compared to some of the other cars out there. I was told this by a pro driver who runs Mugen.
Try to drive both and see what feels more natural to you. You honestly can't go wrong with either car.