I've been looking at this and the sony vaio pro 13, as they are similar. The sony vaio pro 13 is a little more expensive, but I think the extra cost is worth it for the following reasons:
1) Even though the mac book air is light, the sony vaio pro is lighter
2) The resolution is higher (1440 x 900 on air, 1920 x 1080 on vaio pro)
3) Vaio pro has touch screen. Some people may say that is unnecessary, but it is another method of input available and I believe in the next few years it will be standard on all laptops.
4) It's made of carbon fiber, so it's ridiculously strong