Although I personally prefer a chain-stay mount to a seat-post mount the best mount of all in my mind is either an axle mount so that there is zero additional stress on the bicycle frame and the force of towing is directly transferred at the axle end or a custom frame that extends behind the rear tire with a cross-bar behind the rear tire with a lawn tractor type plate and bolt hitch assembly built into the frame. Nutty axles work best for an axle mount towing set-up; basically the hitch point is attached to a metal plate with a hole in it that you slip over the left axle end and the tighten the left side axle nut down onto it. Obviously a dedicated towing frame with built in hitch is a full custom frame job.

As to single wheel or double wheel. Single wheel advantage is it is much better for rougher terrain, disadvantage is that the load in the trailer is added to your balancing load and must be balanced as well. Two wheel trailer doesn't require you to balance the load so you can haul a lot more weight comfortably especially at lower speeds but is not good on rough terrain is pretty much limited to on-roadway or wide path use in practical terms.