Why just these two? There are many better options I think besides these two specific laptops. However, if you are set on one of these, I agree with hpfreak. The M11x is significantly mroe powerful, but you may prefer the Mac OS. If you have no preference to Mac over Windows I'd go with the M11x if it isn't much more.
no real comparison between the two. chalk and cheese. different OS on both. one designed to be superthin and light and the other designed for gaming. have you really thought about what you need the laptop for? because if you just need it for web browsing, word processing etc you will be paying well over the odds for either of these.
With the macbook air's intergrated Intel HD 3000 graphics you won't be able to play any fun games like Crysis 2. So if you plan to game don't ever think about buying a mac and for it's price you can get 2 gaming laptops.