It all depends on the proportion of leg length to trunk length.
I am about that same height and feel most comfortable on a large.
I have a long trunk and short legs so I need the extra top tube length to make the reach comfortable.
I have a mate with the opposite proportions. He was riding a medium with a lot of seat post extension. His latest bike (Air 9 Carbon) he went for a large to reduce the seat post length required. I am not sure how he set it up but I assume he used a shorter stem than on the medium to compensate for the change of top tube length.
Medium is great for tight spaces but even with a 120mm fork is a touch twitchy; however, I use it mostly for racing. I thought a large might be more stable and give me more room to crank on the old pedals. Of course the downside would be a longer wheelbase creating a bike that doesn't do the tight stuff as well. I guess it is all preference. I need to go ride a large and see what I think. Fuzzy thought a medium was the right size as he rides a medium frame.
I'm also 5'11. I ride a large EMD and a large Air 9 and I like it. Before I bought my Jet Carbon I had the chance to test a large and a medium. The large felt heavy, the medium was just fine. So I bought a medium Jet.