From a performance point of view, they are essentially the same.
You could not tell the difference without a synthetic benchmark.
W8 being newer seems to have the occasional problem with some games.
One advantage for W8 is the elimination of the OEM system builder motherboard transfer restrictions.
For me, I am sticking with W7 mostly because the friends and family pc's that I help support are still on W7.
I tried W8 and did not like the tile interface.
For a new user, I might suggest going with W8. In time, W7 will go the way of XP.