Win 8
Pros:
Faster boot time
Overall faster performance by a small margin
Direct X 12 support
A little more secure but being Windows it's not perfect.
Native usb 3.0 support
3d printing support
Native Media file managing and burning support for about all file types.
Cons
Fugly UI which you can change
File system has been changed some like it some dont
Driver headache, not all hardware have drivers which can be a real headache getting things to work and stopping random BSOD's
Win 7
Pros
Great leap from Windows XP in terms of speed
More secure over Windows XP
In works with just about all hardware
Great Driver support
Cons
Lots of downloading to get caught up on updates.