You'll get many different replies for this claiming one is better than the other; but the real is answer is neither. They are two completely different operating systems and were built for two entirely different reasons. Windows 7 is an extremely user friendly and simple OS, because everyone was used to the interface from 98-XP-Vista. Windows 8 has more hidden features, and everything isn't slapping you in the face 24/7. It's a faster OS, and great for mulitasking. However it requires you to relearn an entire operating system... which people usually don't like to do.
My experience is that they are exactly the same OS, the only difference being the start menu. Which is barely a difference really. Especially if you get used to using a keyboard, the process is exactly the same as Win7....press the Windows Key, type part of the name of the application you want, and press enter. Really the biggest change was XP->Vista, and there haven't been any changes that affect productivity since then.
Windows 7, I don't like windows 8. It is interactive and fun to use, however I would love to switch back to 7 now.
I operate both version of windows but I think Win7 is best because of these computer type OS but win8 is like a tablet, mobile type OS.
Win 7 as there are a lot of compatible drivers for your hardware's available. So is stable versions of software liek Steam, Origin etc.