The X has MUCH better battery life, GPS, and reception/antennas.
The digitizer seems laggier than the Fascinate's and the X just doesn't seem to run as fast. Of course, in my opinion, the X's screen SUCKS by comparison.
Fascinate is very rootable and tweakable. The X has an encrypted bootloader, so no custom kernels...but I've read about some "bootstrapping" method that circumvents that. I'm just not sure how much more development that has spawned compared to what's going on with the Fascinate.
The Fascinate dev community is pretty good.