While I own a 135L and think it's a fantastic lens, I'm not sure it's the best way to spend your money right now, based on what you say you shoot and you only currently owning the 18-135mm lens. A more versatile lens would be a telephoto zoom -- either one of the 70-200mm's, the 70-300L or 100-400L, depending on what is more important to you -- reach, speed or size/weight.
I don't mind the weight - ive useed a friend's 6d with a 24-70 mk ii on saturday, and i know the 70-200 2.8 ii is alot heavier, but the setup wasn't really heavy. In fact, I feel that my t3i is too light.
In terms of reach - I don't want to get too much overlap from 70-200, but I guess the 135-200 will be a nice addition to my setup.
speed - obviously I want to get the fastest glass, but my budget is pretty tight.
I know that the 70-200 ii will be the best choice but getting the money for it means that I need to save for another year or two... I can't wait that long