The problem with the Surface Pro 2 is that it's a jack of all trades yet master of none. It's too thick and heavy as a tablet and Metro is nice but there are far too few useful tablet apps. The touch experience really offers very little benefit when the available content is limited. One example--I wanted to sync my Google calendars--sorry, can't do that. The most widely used services and can't sync most with the device. Even Apple, the master of the closed system, allows that. And if you'd like to use something other than IE for web browsing as a tablet? No Chrome, No Firefox, nothing else.
So this is Windows working with everything?
And using is as a laptop is inferior to, well, a laptop. Yes, it has a high resolution display, but in Desktop mode, text is far too small and using the touch interface is terrible. The typepad keyboard is fine, but the touchpad is tiny and nearly nearly as useful--overall not anywhere near as good as as a laptop's and far worse than the MBA's. And frankly, Windows is still Windows. But if you must absolutely use it, you can still do that on a Mac. So you get OS X and Windows, if you need it.
Don't even try to compare battery life. The 11" MBA is quoted as having 9 hours of battery life yet early tests of Macbooks using Mavericks are getting up to 30% more (13" MBA getting 15 hours of battery life vs. expected 12
). The Surface Pro 2 did see approximately a 40% increase in expected battery life...up to 6 hours.
And then there's price. Configure both with 4GB RAM and 128GB and the MBA costs $999 and so does the Surface Pro 2...but you still have to spend another $129 if you want the typecover. And the price difference gets worse if you bump the storage up to 256GB.
The only true advantage I see the Surface Pro 2 having is the Wacom based stylus functionality--no argument here if you need that functionality.
Otherwise, I don't think this is even a contest.