There should be no difference in quality...they're very similar first-level Prestige models, as indicated by the 1xxx model number. Beyond that...
RG1521: This is basically a 2-humbucker hardtail. (model number ending in 2x indicates 2 humbuckers, last digit of 1 indicates hardtail.)
RG1520: I thought this would be like a 1521 with an Edge Pro. I can only find info on the RG1520G, which also has built-in Roland GK output through a
Double Edge Pro trem.
RG1620X: Same as the 1520G, but the Double Edge Pro functions as a normal piezo as opposed to a synth pickup.
RG1820X: New name for the 1620X.
Apart from the 1521's bridge and the extra electronics (and the finishes), they're basically identical.
Basswood bodies, Wizard necks, V7/V8 pickups, etc.
