18" is pretty small. I went in to my LBS today to test out a few for size. I rode a 20.5" Norco Judan and it was very comfortable. The standover clearance was a little small. If I were buying one specifically for anything technical or risky I would drop to the 19" for more clearance. Since this will be more of a winter commuter beater/mild single track bike it's probably better to go with the 21" since it's more comfortable for leg and arm extension. Also, the Judan 21" (20.5 wasn't listed on their site. the must've rounded up) had a 33.4" standover height and the Deadeye 21" has a 32.8" standover.