Gas is much better. This statement is only based on the fact that all competetion shocks are gas shocks :P
On the serious side however, gas shocks have a constant pressure (pre-charge). Meaning they load more even and recoil faster and smoother, giving a better ride. The gas also prevents the hydraulic oil (which is still in there) from foaming around, reducing heat and friction and thus increasing the smoothness.
And they are also better because I just bought a set of gas shocks, so hydraulics can't be better (Or I'll hate myself over it :P)
And as a lil teaser, here's a pic of my new shocks.