I wouldn't bother with the Macbook Air, the 13'' pro is still really light but has much more well rounded specs.
I own one, mainly because they run programs like Photoshop, Illustrator and so forth really well. Plus if the program crashes you can force quit the program without having to shut the computer down.
Depends what you're going to use it for, if you just want to web browse, write up essays and so on, I wouldn't bother. I've had mine for 2 years and still runs like it did on day one. I now have a preference for Macs, but there not for everyone, so do your own research on them first.