I really like the Zenbook and the MacBook Air, and I'm in about the same position as you. While I think that spec wise, the Zenbook is a better machine, the MacBook Air has a fantastic multi-touch track pad, which is hard to go away from. The UX31 however has a better display and better battery life, from what I gather.
The one big downside is that I've heard about the UX31 is that the keyboard is awful. I've tried some display models, and it is pretty rough - keys don't register every so often, and it feels kind of spongy. Otherwise though it's a very high quality machine (but this is of course a big miss).
That said, the new model, the UX31a (also known as the Zenbook Prime) should be out within a month or two, and it promises to be a big refresh of the line (backlit and hopefully much improved keyboard, full 1080p display, etc). Likewise, the Air should be updated soon too, so I would say to wait and see what the updates look like.
The other laptop to take a look at is the Samsung Series 9 - the 13.3" model has the most beautiful display on a notebook, bar none.