Not an easy, straight forward decision to make.
I was faced with the very same decision a few months ago and chose Windows 7 which I like, and am used to, for my new audio computer.
Our XP machine finally died recently and it will be replaced with a desktop also running Windows 7.
For me it's a personal matter, I hate the looks of Windows 8.
I don't think there's much difference under the hood.
If you have any intention of using a touchscreen, Windows 8 is a must.
Dell business machines are still available with Windows 7 (including installation disks).
Otherwise you'll have to buy Windows 7.