If you are going to climb hills, it's not the minimal resistance of the wheel(s) on the road, but the weight of the load. Irrespective of how you carry it, that load will still take the same amount of effort or energy to get to the top.

I haven't toured with my single-wheel trailer, but have carried quite a few heavy loads. including shopping. For the latter, I prefer panniers, really. However, I expect the trailer to come into its own when touring locally on the tandem.