The Haswell Macbook Air is more functional and a much better value at the moment. Another benefit is that you are already accustomed to working with OSX.

The Surface is allegedly more convenient because it's smaller than a laptop, but in reality it's not convenient at all because you have to find a table in order to be productive with it.

If I were going to purchase a Windows 8 tablet as a laptop replacement, I would definitely go with the Lenovo Helix. Unfortunately, it's overpriced and has crappy battery life, like all Windows 8 tablets.

IMO, you'd still be far better off getting the new Air.

EDIT: Another option for a Windows machine would be the recently released Sony Vaio Duo 13. It's got a Haswell processor, a really nice 1080p IPS panel and reviews state that the battery lasts for 10 hours. It's more expensive than the Surface Pro, but it's also a much better machine.