One wheel trailer for many reasons. Having a 2 wheel trailer actually gives you 3 tracks you have to worry about (your bicycle plus the two from the trailer). Riding a road without a shoulder but has rumble strips will be really difficult with vehicle traffic, and those roads are rapidly getting popular in areas. It's nearly impossible to ride the C&O canal with a two wheel trailer. Having a 2 wheel trailer will also increase your odds of a getting a flat.
Reason not to use a two wheel trailer