XP or 7 - MacBook Air Rev. A

7 is a big improvement over XP.

Performance wise, it's probably a little slower, but it feel faster because of the better UI (think OS 9 vs OS X).

All the boot camp drivers for vista work in it just fine.

I have the same spec Rev A. Originally I went with 7 but found it always caused the fans to blow (not good in the library, etc). There was nothing I couldn't do in XP so I went back and have kept it since. Speedier, cooler and quieter works for me.

If you have an SSD, 7. 7 will have built in TRIM support when it releases as well, which helps with the newer SSDs from Intel if you ever swap out for one of the X-18Ms.

HDD, XP. XP is not friendly towards SSDs. I know it from personal experience.

That being said, 7 runs nearly as fast or faster than XP, depending on your hardware. Although if I remember correctly, someone on XtremeSystems posted a thread showing that they got 7 to run on a Pentium II. Albeit verrrrrry slowly, but it ran.

Says a lot in terms of its scalability.

7 is a massive improvement over Vista. And that's an understatement. I was a big Windows user and I hated Vista not because of the whole smear campaign, but because it was a resource hog, and had no real improvements over XP. 7 has the speed of XP with the upgraded UI of Vista (only better).

So, that means, MS managed to improve the UI in 8 years?

Neither, it'll make your MBA feel 'dirty'.