With the lever I have better feedback as to whether it is seated correctly. IMO both are good, and both require that I pay attention (easily said, but not always practiced by me).
I tend to shoot a fair number of vertical comps, and often shoot a scene horizontally then look for something vertical I can use from the scene (or visa-versa), for which the lever style clamp is a life saver. And further, I prefer the way the RRS lever tucks under the Mamiya body. Really I like both, but I do find advantages to the lever-type at times as well. I just have to be careful with either.