hey soccer,
I'm looking for basically the same answer to the question you asked... Here's what I've found out so far though... X-Plane is more realistic if you are a pilot, whereas FSim is somewhat better if you're just mucking around. In other words, x-plane offers more realistic malfunctions, more realistic cockpits, and better IFR scenarios (approaches, SIDs, etc..) The flight college that I used to attend once used FS, but have switched to X-P as they have found it more realistic in the true sense of flying an aircraft, with a focus on IFR flying.
My beef right now, is that I'm still trying to figure out if the newest version of X-P will work on a Macbook, which has only integrated graphics, instead of the actual dedicated graphics card found in MB Pros. Also, will X-P work natively with the new Intel chips? Or will it have to use Rosetta in order to run it??? This is something that we really need to know in order to get the proper performance out of the program...
Cheers,
GS