After what has seemed like an inordinately long delay, the Really Right Stuff baseplate for the Z9 arrived yesterday. I popped the SmallRig plate off the camera and shot them side by side:
They are both well made. If you need a QD connector, get the RRS piece. If you want to save some money, go with SmallRig. If you do decide to go with the SmallRig, I recommend buying the screw kit from them and replacing the slot head 1/4-20 screw with one with a hex head for an Allen wrench.