I feel like it really depends on the person. I have tried Windows 8 and I feel like it is meant more for tablets or touch screen monitors. I have been comfortable with Win 7 and feel like im on a computer instead of a tablet when I use it. It really depends on the person and what type of layout they like for their OS but personally, I would go with Win 7.
I would say, at this moment to go with Windows 7 simply because driver support is better and you are less likely to run into issues there. That being said, as time progresses, this may become less of an issue, especially since Windows 7 and 8 have pretty similar code when it comes down to it.
I say Windows 8, its the newest OS so it will have support for longer. There's no downsides to it now (you can get 3rd party programs to restore the start menu and bypass Metro entirely) and it performs the same as Win7 in games and has faster boot times.