Unless you live in a state with a magazine capacity limit, buy the new one. Beretta mags are everywhere. The new ones are indistinguishable from pre bans. During the AWB, they were marked LEO USE ONLY, but they are not anymore. You cannot tell the difference between a mag made in 1993 and one made last week. Floorplates are often changed out even on older mags. And $650 is too high for a new Beretta.
HOWEVER, if the used gun is old enough that it has the metal trigger and guiderod, AND is in comparable shape as the new one, I would get the used one. The plastic trigger and guiderod bugged me and I spent $90 to replace them on my M9.