X-Plane is considerably less hardware intensive, but it is for a reason - as I said before, there's a lot less going on in the background.
Regarding how things fly between the two sims, as a pure numbers sim X-Plane is probably better. If real world calculations on an airframe say it will stall at 53.28KIAS, that's what it will stall at in X-Plane. However, I've always found that it is too "clinical" - real aircraft don't stall at exactly the calculated airspeed because it differs in surrounding air conditions.
Are the flight conditions "more accurately modelled"? My answer would be no. They are differently modelled. You can still do impossible things in X-Plane and, in particular, rotary wing and mixed-mode aircraft (eg BB609, Osprey) are exceptionally unrealistic in X-Plane. Sometimes, in both FS9 and FSX, I've got very tied up in controlling the aircraft, particularly doing bad weather approaches or hand flying blind. I never felt that in X-Plane. I was always playing with a simulator, it seemed a lot less intuitive and had a lot less "draw in".
Given a choice of sim, if I couldn't use FSX, I would recommend FS9 over X-Plane still. X-Plane is developing well, but I don't think it will ever catch, let alone pass, what the far bigger team employed by Microsoft achieve.
The version of X-Plane that is used in serious simulators (not commercial ones - they'll be GA sims as X-Plane really doesn't simulate airliners at all well) is not the "publicly available" one either. It's more like the difference between FSX and ESP, where the version of X-Plane you'll be getting is FSX.
Ian P.