I had a medium, now have a large. I'm (5'11"ish)
If you search the forum there is a good piece by Dave Turner re. sizing. General recommendatoin is if you are in between sizes err on the large.
I bought the medium coming from a medium SC Heckler. However the Turner just didn't feel right.
The problem with the medium is not the top tube but the head tube and, to a lesser extent, the seat tube. The short head tube means that, at 5'11" you will probably need a stack of spacers. You will also need a whole heap of seatpost to make it comfortable.
Of course much depends on body shape but I would say go with the large.
HTH