I have both the Marlin mount and the Sumtoy mount. The Marlin is flat bottomed, while the Sumtoy is rounded on the bottom to better match your gun's receiver. Other than that, there isn't a lick of difference in the two. I'd pick whichever one was more easily available or cheaper.
I went with an EGW picatinny rail.... My rationale was that if I didn't like my vortex venom, it would be incredibly easy to switch out to another reflex sight or even a scope.
Since I LOVED changing to a compact reflex sight and see no reason to ever change, I wish I had gone with the sumtoy mount. I do like the fact that my venom is mounted much more forward on the picatinny rail than what the sumtoy mounts allow for.