It depends on how you have it built. I will give you an example.
My 8.8, which had a locker, but stock 31 spline axles, was 190lbs. The Currie 9 inch I replaced it with was 185 lbs. However, I had the 9 inch built with a spool, but also 35 spline axles, and an aluminum section right behind the 1350 yoke (its not the carrier housing, forgot what they called it).
Anyway a lot of people think the 9 inch is a lot heavier when actually its not, depending on how you have it built. The real question is, do you really need a 9 inch vs just having an 8.8 built.
I like the 9 because its bullet proof, and no need for c-clip eliminators because of how the axle bolts up. The 8.8 you need an "axle retention device" or c-clip eliminators to be legal at the track at 10.99 or quicker.