I'm 5'7 and had the 18" on my RK, absolutely loved them. Hands just above the shoulders. Of course it seems to me like the top of the seat was closer to the top of the trees on the RK than it is on a typical softail...
I'm 5'8" and I'm running 14's. It all depends on what seat you are planning on running too. With my C&C seat you can see that it puts the bars right above my shoulders. If I had gone with 16's I would hate life in parking lots, but 14's were perfect for me. And they do run a little taller than advertized.