Windows 7 or 8?

depends if you have alot of programs that are not compatable with win8(very few) or you are too attached to traditional windows layout(which you can mod windows 8 to be similar to). IMO get 8 for its quicker boot up times and will receive more recent updates from microsoft, but others will say stick with 7.
 
Windows 8 is superior to 7; except for the interface.

You will need to purchase start 8 for 5 dollars, or use classic shell which is free I believe, these will restore the normal desktop interface and hide metro for good.

There is zero reason to purchase windows 7 over 8 as long as you have Start8 or Classic Shell.

You're very unlikely to experience compatibility issues.