theodoristef :
Is there any performance difference between 7-8.1 ? I have heard win8 is more cpu friendly
Well this is a pretty subjective question, but I'll weigh in.
Depending on what software you are running, Windows 7 and 8 / 8.1 run pretty similarly. I have seen a few comparisons between Windows 7 and Windows 8 under gaming and they either tied or Windows 8 came out on top slightly. Beyond that MicroSoft significantly improved boot times in Windows 8, so if that's something that turns you on, then I'd say it's worth it.
What turns people off is the Metro interface or Start Screen, whatever you want to call it. I am sure you could get used to it, but most people (me included) download Classic Shell and boot directly to the desktop with a fully functioning Start Button.
I was on the fence for awhile, but I thought it's the newest and in the long run have the longer support cycle, so I might as well jump on board now.