I'm with a rear hub, as the motor is pulling the trailer directly, then. There's no additional stress on the bike no matter how heavy the trailer, and it doesn't matter what gear you're in, the hub motor is always in the right gear to pull the trailer (such as a wheel dropping in a hole, or a sudden step/object to climb).

My Phillips Cleveland trailer, which i've modified with alloy leaf-spring suspension, now hitches directly to the 14mm rear axle (it clamped onto the chain-stay before, which got in the way of my foot/power-cable/disc..).