This might help you make your decisions.
Win7 - Familiar interface, has the ability to emulate WinXP (via an
embedded VirtualPC VM) for non-3D applications (i.e., probably not for gaming) if you purchase the professional edition or above.
Win8.1 - (there was no Win8, there is only Win8.1) newer, more confusing interface BUT new licensing model means that you can do something you couldn't do with Win7 and previous: If you purchase the OEM version of Win8.1, you
are allowed to move it to a new machine, provided that you are the owner and operator of the new machine (and remove it from the old). Previous versions of Windows would force you to purchase a new OEM copy for your new machine.
Testing shows
Win8.x is actually slightly faster than Win7 in gaming, if that also helps. Also, Win8 will no doubt be supported for longer.
Personally, Win8 is making a good case for being your next OS, even if it means steeling yourself for the new interface.
I will be using Win7 on my machine, though...but I am doing so specifically for all that backward compatibility capability I will get (and the fact that I already own a copy). We will see whether my
next machine earns Win8 or Win9.
--Patrick