you cannot compare a 70-200 with a 50mm lens. They are not the same, even with the same aperture, they wouldnt be.
the 50mm is mostly for portrait or macro (with ext tubes) while the 70-200 can be used for anything that need reach other than wildlife.
I own a 70-200 2.8L and its a great lens, but if I had to go back in time, I would have taken the F4 and a 400mm 5.6L prime.
Anyways, to be back on the subject, there isnt any real advantages for you to upgrade right now. If you want to improve image quality, the best trick is to improve your technique, then improve your PS skills, then improve the lens then improve the body (get a full frame).
Go into that order not the other way around.