I started off with a perfectly serviceable set of Shimano Deore XT brakes. But after riding some of the downhills on the trail, I felt I’d need something more robust for when I was trying to control the trailer on those hills. I replaced them with Hope Tech 3 E4s, which have four pistons per caliper (rather than two). I also increased the rear disc from 165mm to 180mm (same as the front).
I (and by “I,” I mean “we,” and by “we” I mean Rick Jorgensen of Bicycle Engineering) considered going up to a 205mm front disc. But by sticking with 180mm, I only need to carry one spare disc on the ride, as it will fit either wheel.