I have no experience with Hunter Derbies, but I've been on a LOT of Hunter Paces and they are SO MUCH FUN!
But they remind me most of the cross country portion in Eventing, nothing like Hunters. You're not judged on how you look at all (like Eventing/very unlike Hunters). In my area, the local hunter paces have 3 divisions...Jumpers, Hilltoppers, and Trail Riders. The Jumpers usually have a faster optimum time and they must jump ALL obstacles on course (usually between 35-40). For Hilltoppers which is what MOST people around here ride in, the jumps are optional so it's still at a fast pace (mostly trot/canter), but you get to pick and choose which jumps to do so you can go around any that you want to.
Trail riders are at a leisurely pace with no jumping and they actually aren't in the running for the ribbons on our local circuit, only the Jumpers and Hilltoppers. It's only $25 per person here, and you go out in teams of 2 or 3.
I love love LOVE hunter paces, they are a blast and so laid back...no worries about strides, form, equitation. Just you and your horse and your friends in open hunt country doing what HUNTERS in the original sense were meant to do I would highly recommend them to anyone interested!