Acrylic will need a much thicker piece below the acrylic, or be milled form a thick slab and that will not be cheap compared to some laser or water jetted aluminum.
Aluminum and brass will have less ping than stainless but be less stiff.
Is there such a thing as too stiff?
I tend to take the sports car approach to it, the chassis should be stiff and all the suspension/switches should be doing the actual work. Why? Because if you rely on the plate your feel will not be consistent across the keyboard. As such, I don't mind aluminum or brass for smaller keyboards but larger ones may need steel to span the gap, however, how the plate is attached can be every bit as important as material.
Take the two plates in your picture, the silver one has a plate that mounts to the case, this should make it nice and stiff as it can't really move. The brass one (Poker/GH60) however is not mounted to the case and actually floats, being held aloft by the switches, no matter what you use, you will NEVER be as stiff as the silver one. While it may hold the switches level with one another, it's still being held aloft by the switches and ultimately the pcb, a thin, flexible pcb, with inconsistent mounting holes, some areas will have more flex than others. Basically if it's not mounted well, it won't matter much, the feel will not be good and if it is mounted well, then the material matters less. And yes, I know they do feel different and sound different, but if you don't have a good foundation that feel is compromised from the start, hence the race car design theory.
Personally, I would err on the stiff side if you aren't sure about how stiff it will be.
