Win 7 if you are going to play older games. I've read some threads where people are having some issues with a few older games. Win 7 fixes do not necessarily work for Win 8. For example, I have yet to read any post about someone successfully running Fallout 3 in Win 8.
If you plan on playing older games then Google for compatibility issues.
Toms did a benchmark on this very topic. Frame rates are slightly better in windows 8, probably due to the fact that it is a lighter OS than 7. I don't know about the compatibility issues, I haven't run into any, but then again I haven't played fallout in a while. But the review say's that they ran into compatibility issues with sleeping dog's. But as they say in the review, it's likely to be fixed by a patch soon.