Single wheel trailers work better on rough roads and trails.
2-wheel trailers can flip. Most of the tourist trailers I see are single wheel.
On decent roads, 2 wheel trailers make better cargo carriers, they can cope with long and unusual loads. I'm about to pull the trigger on a Carry Freedom trailer for transporting stuff around town.
You only need touring trailers if you are touring with a family or need to carry something big like a few days supply of water. Solo tourists are probably better off with rack and panniers.