Now that I've made a decision (alu) can anyone explain to me the difference in plate materials?
Previous versions of LightSavers are tray mount, where the PCB is screwed into the base so they were essentially a PCB mounted keyboard with the plate aligning everything. The difference in materials is mostly be for looks and sound. You could even go plateless with PCB mount switches.
The LSV3 is top mount.
The plate is be mounted to the case, leaving the PCB suspended by the switches. This is where materials matter. Acrylic that is a uniform 1.5mm thickness is unsuitable, polycarb and aluminum are more forgiving, while steel is going to be stiffer and have more ping. Most people agree that steel would be best for linear switches but may be too harsh for switches with sharper tactility like MX clears, MOD and Zealios. Its preference.
EDIT: Sorry for the initial inaccuracy