If the price of the new one after rebate is within $100-$150 bucks or so of the used one, calculating magazine number and magazine capacity, I'd go new.
H&K increased the mag capacity of their magazines a couple or three years ago by two rounds per mag. If the old one and the new one have the same number of mags, but the old one has the previous gen mags, I'd go new.
Previous P30 V3 and LEM owner here, son still has his P30L V3 .