The difference is the RRS clamp is a quick release lever clamp. The standard Markins clamp is a screw one. It is much easier and faster to use the QR lever clamp to get your camera in and out than the screw one. Screw clamp to screw clamp between Markins and RRS there is some difference but not much.
If you mean of the Markins lever, then I would presume it's a decent lever albeit it rather new without a lot of history. The RRS clamp has a long track record, is well regarded, and has been successful.
The Markins clamp, I believe it does have an adjustment to accommodate the slight differences between plates from different manufacturers. Lever clamps are more sensitive to those variations since they don't adjust as widely as do screw clamps. That said, if you use any lever clamp you should probably constrain your plate purchase to that of one manufacturer or at least verify that the specific plate you get is compatible with the other plates and clamp you have.
Some, me included, don't like the idea of an adjustment. It's just one more thing to fool with when I'm in a hurry if there is a problem.