I'm 5'9, and vary between 155 and 170lbs, and have big feet. I've owned both sizes and fit fine in both. I prefer the large because it's faster because it rides higher in the water. On the flip side I have to muscle it around more to turn. The medium handles like a dream, and will still do okay loaded for overnighters, but when it's heavy and in big water the medium will feel a bit more squirly.
I would go with the large and just spend some extra time padding it out. The outfitting is great. Easy to add shim pads (I have a good 1.5" extra on each side) behind the hip pads. Foam blocks on the outside of your knees can help hold your legs against the thigh braces. I've worn full size Keen trekking boots inside my boat, and they fit safely.
Can't go wrong either way. It's a sick boat.