I have a 7.3FX in the 17.5" size. I'm about 5' 11" and the ride is fine for me. Had to raise the seat up but I'm mostly comfortable. You would probably be fine with either but you say you are comfortable on the smaller frame so that should give you your answer right there....probably raise the seat up like what I did. A word of advice on the smaller frame, it only has mounts for 1 bottle cage (on downtube). I believe that the 20" frame will have 2 - downtube and seattube mounts.