Hi Carl, I have the RRS monopod head for over a decade now and still very pleased with it.
To my knowledge both the RRS and Kirk heads have one knob to set the drag friction and to lock it whereas most of their tripod heads have separate knobs for locking and setting the minimum drag.
The RRS head has built in arca plate on the bottom so it can be used as a poormans alternative for a Wimberley head when mounted on a tripod head where you can combine the panning function of the tripod head and tilting function of the monopod head.
Lately I've changed my tripod head to one with a limited tilting range, the monopod head can be used to extend that range sideways for portrait mode (when you forget the l-plate) or to shoot straight up or down. Attached a quick snapshot of a possible setup for portrait mode.
Whichever head you decide to buy my advice would be to get one with a screw clamp and have the security screws mounted on your lens or camera plate. On the first NG safari my 200-400VR was hanging on one of the little security screws because the RRS lever clamp couldn't keep the 200-400VR with RRS plate in place when I was walking around with the monopod swung over my shoulder
Needles to say I replaced the lever clamp with screw clamp shortly after the almost fatal event.