Having tried both bikes due to mates having them, i would definietly say i would prefer the giant.
The guy who owns the giant got it on an amazing deal. £1400 for a bike that cost upwards of £2500 (not sure of exact retail price), brand new! I think part of the reason that it is such a good bike was the price tag. (Sram xo, DHX air 4, 36 floats, race face finishing kit etc)
It is very light compared to bikes of a similar nature, acceleration is very quick (not something i ever thought about or that i needed until i hoped on this bike...and then decided my bike needed to be the same!) and it is smooth and controlable in the air. I havent taken it down any proper DH, but i can imagine that it would cope perfectly fine, suspension reacts smoothly to hits. Oh, and it glides uphill too.
The sx is a little heavier, but, it will probably survive the rough stuff a little better than the giant. It feels smooth and easy to handle. Seems to have a low centre of gravity too. Again, handles well in the air and on the tight tech stuff.
If i had the cash to splash out on either, my money would be on the giant though. Such a nice bike. Infact, i kinda regret splashing 2.5K on a bike that is only midly better than one for £1100 cheaper....dammit!