XTR is the best on the market to be honest if you want lighter youll have to go for the shimano dura-race which is what are used on road bikes but not much differnt to XTR. and it lasts a long time im still using the XTR cassette i bought for my downhill bike 5 years ago. but it depends on what you'll be using the bike for as XTR is for race bikes and is feckin expensive too.

i dont think u got any chance of getting 7 speed tho. even 8 these days is a hassle to get hold of. 9 speed is the way.