A little depends on personal preference and a whole lot depends on the dog's body shape. You did not mention the breed(s) you are working with.
I race with Airedale terriers who have a short, square body, and I have found that H-backs work well for most of them, though I have one dog that uses another type (can't remember the name, but it is shorter then the H-backs.) Most common is the X-back.
Sally J. Dawson and the Mushing'dales N1BCF; "RED HAT" Musher Live each day as if it is your last
In my variable climate I prefer smooth-surface harness filled with closed cell foam (not a fleece surface). I got ice balls in the fleece. I have had many brands, no particular preference other than smooth surface.