I've been pondering this too, ever since my old Dell Desktop refused to boot recently (re-seated memory chips and cleaned crud out CPU heatsink, then it worked again fortunately).
There is no question in my mind that I would get Windows 7 Pro. I can't see where Windows 8 would do anything but annoy me. Now I have a Slate 500 9" tablet with Win 7 Pro, and I've actually thought about updating that to Win 8, because Win 7 is not much fun on a tablet. Not sure how well it would work on this tablet though, so I probably won't bother.
Now I may a bit different from you however. I have always been a Mac user (since 1985 and Apple ][ before that). I only use Windows for mapping/GIS software and am not interested in managing media or running any "productivity software". I use the Mac for CAD, audio/video editing, e-mail, photoshop, etc.
So I just want my Windows machine to be fast and work well with Globalmapper, cgpsmapper, GPSMapedit, TypWiz, etc. I have never been a fan of the Windows user interface, but I'm used to it and Windows 7 offered some nice refinements.