There is some learning curve with Windows 8 (weeping and gnashing of teeth, mostly). Win 7 is quite stable, will remain serviced for quite some time, and is an easy transition to functionally coming from XP.
Win 8 works fine with all of my software and hardware, but I did my homework on that first. The biggest thing with Windows 8 for me is just dealing with the whole active tiles thing with their new user interface. I just bypass it and go to the desktop, with most of my common applications pinned to the task bar.
In Windows 8, since there is no longer a start menu, you can get to your applications by hitting the Windows key, then just start typing the name of the application you want to run, and it will automatically present search results. From there, you can decide if you want to just run it, or you can pin it to the task bar, or you can make it an active tile and find it there.
I do not know of any major performance benefits or costs between Win 7 and Win 8, so choose based on homework results for drivers and such, and choose based on a sense of how you will like dealing with the changes that Windows 8 brings.
Compared to XP, Windows 7 is like a race car versus a bicycle. Stable at speed and a joy to drive, versus getting passed by everything and wondering if you will fall off at the next turn. (And I REALLY was a die-hard XP fan).
Bob Bone