The single-wheel trailer like the B.O.B. has one advantage over the two-wheel type off pavement. As long as you're on a smooth graded path they're probably equal but as soon as you encounter a two track road, like most of the C&O Canal Towpath that I've seen, the single-wheel follows your bike in the path it describes while a two-wheel is going to hit the rough spots that you and your bike are dodging. I rode north and south from Marlinton years ago and do not recall the trail's surface but I recently ran into a fellow on the C&O and I asked him about his two-wheel trailer and he said it was a pain in the ass in the two track. He picked his line and the bike rode fairly smoothly, but if there was a bump or hole to the left or right the trailer hit every one.
I still had the pic! The guy's a veteran with a prosthetic leg, his dog in the trailer and enough gear to do the whole GAP/C&O. Note the trailer wheels and they're on fairly smooth grass in this photo. Imagine rough trail.
