I recently bought a Dell XPS13 and an ASUS Zenbook 13" for different people at work. Their both pretty nice, but for costing almost the same (I think the Zenbook was $50 more) the screen on the Zenbook is much better than the XPS 13. On other specs, I think they’re identical. The Zenbook also comes with USB to Ethernet and VGA out dongles, while the Dell does not. Those aren’t exactly expensive items, but it’s nice they’re included. So between those two I have experience with, I’d recommend the Zenbook over the XPS 13, unless you find a great deal on the XPS 13 and don’t care about the lower resolution screen.
Apparently there are trackpad issues with the Zenbook. The one I setup was just fine, but supposedly I was just “lucky.” From what I’ve heard it comes with trackpads from different manufacturers, and some of them are unusable. The same also applies to the SSD. It’s a gamble as to which manufacturer will have supplied the SSD, and some are fast, and others are not so fast. This is all stuff I’ve heard, but not experienced, so I can’t confirm it.
I can confirm that the trackpad drivers for the XPS 13 that Dell puts up on the XPS 13 download page do not work right. Hopefully that is just a mistake on Dell’s part, but I’ve not been back to look as I managed to get the trackpad working properly with some old drivers suggested by a forum. BTW, the trackpad on the Dell did work properly with the preloaded Windows Home version, but I have no idea which drivers those were. I upgraded the computer to Windows Enterprise, so had to reinstall the drivers.
The MacBook Air is also an option. If you want to run OSX get a MacBook Air. If you don’t want to run OSX, then don’t get a MacBook Air. (Yes, the Air will run Windows and Linux, but there are cheaper options if that’s all you want to do.)
If you need to buy now, then you need to buy now, but if you can manage to wait a month or two, then Intel’s Ivy Bridge processors should be available, along with the new ultrabooks carrying them. With computers it’s (almost) always better to wait to buy, but when you know it is only a month before the new products hit, it might be worth delaying, even if you’re ready to buy.