I played through both versions twice to see the difference in both performance and graphics.

The 360 version had better antialiasing and sharper textures, but it wasnt a huge huge difference to me unless i was standing still looking around, going out of my way to spot a difference.

The framerate however is where the 360 pulls out ahead however... 

Both versions of the game have the occasional momentary stutter (presumably whenever it creates an auto save for checkpoints), and both versions occasionally have slow downs, but the 360's were noticably less pronounced than the PS3 ones.

This was mostly apparent in the mech piloting section at the end of the demo, where the ps3 version was a tad choppy in parts. That all said tho, it is not at all game breaking for the ps3 version. Having played through Condemned 2 (monoliths previous game) on the ps3, and hearing about how bad the first fear game was on ps3, i can confidently say that Fear 2 is the best optimisation job monolith has done on ps3 so far.

Based on the demo If you have both systems i'd recommend getting the 360 version for the better frame rate and slightly better graphics. But if you only own a ps3, there is nothing gravely wrong with the performance at all. It was perfectly playable and the drop in framerate was no where near breaking to my enjoyment of the demo.