I've owned both, and would recommend both.
I kind of preferred the fixed 200mm, I felt that, for stills at least, it was the faster lens to use. This is as simple as, I couldn't waste time zooming, so spent more time shooting.
It's less flexible, and 200mm/320mm is going to be too long for some situations.
I loved this lens because it made my humble XTi really come to life. It was exceptionally fast focusing with centre point AF / Ai Servo and the lens focus limiter on. I got the genuine canon tripod ring as well which made the lens handle really really nicely on a monopod as well.
I think at 200mm the prime has a very slight optical edge over the 70-200 f2.8L, but then the 70-200 offers you all the other focal lengths and is excellent accross the range.
I really love using my 70-200 f2.8L, it was really an essential switch for me as I use my gear for video, and have no real regrets.
If I was buying for stills only, I would go with the fixed 200. I only had it short while on my 7D, but what a combo!