One vs. Two wheeled bike trailers
One-wheeled trailers are more agile and don't increase the width of your bike.  They lean with the bike. If you're going to go really narrow places, you need one-wheel.

Two-wheeled trailers can generally carry more weight and are less prone to making the bike fall over when you stop.

So, what are you going to do with the trailer?  Going up and down mountains with narrow trails: get a one-wheeled trailer. Carrying 50kg of equipment from the hardware store home using streets and wide paths: get a two-wheeled trailer.