I found them pretty much equivalent also, when it comes to performance. The X9 seems to me easier to install and adjust, also stays tuned longer. The XT is a solid value when it comes to durability, though. I like the SRAM triggers better, but that's personal, performance wise I would say they are the same as Shimano's. Some people complain about the low feedback from the springs in the SRAM's that can lead to overshifting. I can say that it is true that you can hardly feel them click, so far not a problem to me (note I am on X7's not X9's) I went with SRAM because I wanted to try something different from shimano, and so far I am not disapointed. But I can't speak badly of the XT. At least not too badly.
If you are tight with money, and your bike is 9spd already you should consider upgrading just the rd to XT, and save some dough, and if you are running a deore fd, you may want to spend an extra $30 and get a SRAM Xgen or shimano XT. It will probably improve the front shifting a lot. If you stay with your old shifters, replace the cables anyway, they don't cost a lot and rusty cables can cause a lot of headaches.
Finally, if your cassette/chain are old, you should serioulsy consider replacing them. You are never going to get good shifting with an stretched chain, and an stretched chain will damage your cassette. And a worn out cassette will damage your chain.