How does XPlane compare to FS9? Look at my signature for my computer specifications, tell me if switching from FS9 to Xplane 9 would be a good idea in my case.
Thanks!

Short answer;

They are both entertainment software, neither one is all that realistic.
Use the one you feel most comfortable with

I've only flown a Cessna 172 in real life, so I base my comparison on this aircraft alone.

In my opinion the FSX version feels far more realistic as to handling, turns, etc. X-Plane has always felt to jittery on the controls. Rolls seem too fast, elevator effectiveness a little too responsive. I can't really put a mathmatical finger on it, it's all feel to me.

Plus, FSX looks good. Visually, both have drawbacks. FSX world is too cartoony on a clear day, X-Plane is too dark.

FSX = Win. But I'm glad I have both to compare. I love the Cirrus Jet in Xplane. That alone is worth the price of admission.

^^ Agreed.

One thing, however, I do love about X-Plane is the flight dynamics in certain situations and aircraft. For example, the X-15, Sr-71, V-22, F-22, and X-35B have flight dynamics that are way beyond anything FS9 or FSX could accomplish. Then there's Lock On Flaming Cliffs 2.0, which kicks ass also. If you only fly general aviation, then stick with FS9 / FSX.

Some problems with the FSX flight modeling occur regularly, and so I can understand the frustration of some sim pilots. My pet peeves:

1) The need to hold rudder while in a turn to maintain ball-in-center coodinated flight. If the plane is trimmed properly in level flight, a moderate bank should not require continued rudder input. Rudder should only be required to counteract the adverse yaw of aileron deployment in such conditions. This trait is so common I wonder if it is possible to get rid of it.

2) Slipping- Most FSX aircraft do not slip in a reasonable fashion. They don't substantially gain drag during a slip, and heading control is unrealistic. As a positive note, the Extra 300L in the Acceleration add-on slips much more realistically than the previous Extra 300L.

3) Stalls and spins- Much work remains to be done here.

Well, I have a bit of flying experience in real-life plus a maintenance engineering background.  I tried X-Plane 8 way back and deleted it off my hard-drive within the first 5 minutes because it was graphically inferior to my MSFS, the controls were--as someone already mentioned--jittery, and the lack of many quality planes. 

Having flown Flight Simulator for oh... I don't know, over 15 years, I tended to be a little biased.  I recently picked up X-Plane 9 in realizing this and decided to give it an honest try, and quite frankly, I wasn't disappointed. 

Setting up XP9, the controls and the like wasn't easy.  It was still very jittery for me at first, but after spending some time playing around the settings, I found something that quite accurately translates my little joystick relatively close to what I can accomplish in FS9/FSX, and perhaps even better.  When all was set up, and when I finally took the C172 (which I have flown for hours in real life), I really began to appreciate the flight dynamics.

I used to think the C172 in FS9 and FSX was pretty spot on, but it took a bit of flying around in X-Plane to realize that FS really lets you get away with a lot of things.  There is a slight feeling that seems to be lacking in FS as opposed to X-Plane; from flaring the plane to the little adjustments I need to make when trying to center-line my aircraft onto the pavement, and even the effects of the weather.  There are some things that X-Plane has which, in my opinion, is totally lacking in FS.  I know some of you can jab at the flight dynamics all you like, but there is a reason why even in X-Plane, there are so many real life pilots who fly it.  Perhaps it just takes a bit of tweaking--at least in my case. 

Out of the box, I think X-Plane looks better than FSX in scenery (I always found the latter looking a bit cartoonish.  In heart though, I'm more biased to FS9 over the two sims;)), though again it lacks in aircraft quality and almost every airport in the X-Plane world lacks objects.  Also, the autogen sucks, but I can easily forgive because the stock 60 gb worth of scenery does look quite reasonable.  Quality addons are emerging, however, and I recently had a taste of some of XP9's higher-end payware, which I absolutely -love-.  X-Plane also has its own share of freeware photoscenery, which puts it right up there with FSX. 

X-Plane is evolving, and quite frankly, I'd recommend it to anyone who can fork out $29.99 (thank goodness its that cheap) and are into flying in general.  It may not have tubes like PMDG or Level-D at the moment, it may not have much for traffic, and some of its default flight models are terrible, but I have come to recognize that the sim excels in a LOT of different areas that is not immediately recognized had I not given it the chance it deserved.  With X-Plane 10 coming out sometime by the end of this year, there may be more on the horizon.

They are very different, neither totally realistic, and both are fun...