My 24-105 is parked on my 5DmkII, but my favorite zoom is the 100-400 though I find I am using it mostly at 400. Perhaps, I should get a 400mm prime!
This is almost my situation exactly except on a 60D. By far my farvourite zoom is 100-400 used 95% of the time from 300-400. But I can understand if you live in NY City why a 50mm could actually be MORE versatile.
If I could justify owning (spending the money) the 400 2.8 I would but the 400 5.6 prime just does'nt offer enough for me to trade in the 100-400.