Which Windows-Version for MS Office?

Which Windows-Version? (MacBook Air / MS Office)

Hi guys

I would like to use a recent version of Microsoft Office (e.g. 2007) on my MacBook Air (MC503LL/A with 4GB RAM and 128GB Flash), where I am using OSX Lion. About 85% of the time I use Word and about 15% of the time I use Excel. In the future, I might possibly try Visio but this is not for sure.

So my question is, which Windows-Version shall I opt for, XP or 7? (Note: Vista is not an option here)
First-hand experiences would be valuable here...

Thanks a lot!
Cheers, Andy

Edit: I am planning to use an external monitor (probably 22", 1920x1080)