I have both and i can tell you that in my expeirence so far that x9 is better FOR NOW.
In x9 you can get mods witch allow you to do all sorts of things, its x9 so that means everyone can use it, you dont need a great computer for it, and because more people use it there is alot of support.
Although in x10 you do get stuff like awsome graphics, particles, darkAI, bloom, water, instacing, ragdolls, all of that stuff but x9's mods cover most of that. And unlike most games that have an option of x9 and x10, FPSC x10 is just x10. But you do get lots of problems with it like some model packs not working (TGC working on it), lack of support, and the engine itself has lack of polish and flexibility. But i think that soon lee will be releasing the source code for x10 (hopefully) so that means MODS

. And X10 makes use of multiple cores unlike X9 with the real lack of performance (main let down).
And if you comp cant handle X10 UPGRADE!!!!! I dont know why people dont just buy a GTX 270 for example, keep up with the modern times and enjoy the latest games at there best, it really is worth it.
So in conclusion i say go with X9 for now but X10 users will rise and bite back the X9 users in the A$$.
Tomorrow never comes until it is too late...