Of those choices, I'd probably choose Win7 since that is going to be supported for the longer-term. Either one will probably be just fine for that workload once you strip away all the extra cruft, trialware, etc.
For what it's worth, I'm currently fiddling with an Acer netbook and Ubuntu 9.10. Very snappy combo. It's going to be used mostly for watching movies on long flights to Asia and the usual web/email schtick. It came with XP, but it felt a little sluggish to me. In all fairness, I didn't really spend a lot of time trying to figure out what the problem was and just decided to pave over the hard disk with Ubuntu.
Best,