I bought FSX. Luckily, my comp is powerful to hold both, but FSX is a lot slower, because of all those cars, and moving traffic around the airport. It's a good feature if you have the right specs.
I would say, stick to FS9, and just think it over, and read a few reviews. All I'm going to say, is that FSX is a brilliant computer game, which has state of the art graphics, and amazing textures...if your comp is strong enough.