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.
8. Does everything 7 does, only faster and takes less space. i.e. great for installing on an SSD...and it's supported well into the future, plus well less updates to install, and you can easily refresh Windows without losing your Data...
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.