I am bout 6' 1" and a few months ago bought a Large 2006 Specialized S-works 120 FSR frame.
It has the laid back Thomson elite seatpost and a 100 mm Thomson X4 stem.
The FSR seems to have a shorter top tube. I have 2 other 19.5" inch frames and would say that the FSR
seems a little more "compact" in the reach dept. It seem to have a larger Drop to the handle bars as well,
feels a little racier. But it still fits fine, and the FS sure rides nice.
At 5' 11" I would say a medium would be on the verge of too small, especially if you have a longer torso and shorter legs,
I would ride one first.
Cannondale Six5, Specialized Stumpjumper FSR & old Hard Rock
I'm 6"4 riding an o4 Stumpy FSR with an XL frame. My I have a 30 (?) mm offset seat post & 120 mm stem with the seat kicked up pretty good. The bike is a very comfortable ride with this setup, but believe I'm at the edge of the spectrum with the XL size frame. That being said, It sounds like you are taking a gamble going with a medium.