I had the same dilemma. Ultimately I went for the 56mm for the shallower depth of field. I know if I figured it would be cheaper in the end as I would have ended owning both. I tend not to sell lenses once I have them.
My current set up is
23 f1.4
56
18-135
Not sure what my next one will be. I might go for the 16mm or the 100-400 or perhaps wait for the 8-16 when it comes out. I'm happy with this set up for now.