Most used Glocks tend to be priced higher than what I am personally willing to pay, so I would go for new because they tend to hold their value to a point where the price of new vs. used is often not that much of a difference.
If you are looking for a Gen 4 G19, the price difference between a good condition used 19 and a brand new 19 could be under a $100. Even older generation Glock's hold a high value, especially in standard LE-grade calibers such as the 19.
(edit: with that said, I've seen killer deals on Glocks in .45 GAP, 10mm, and (to a lesser extent) .357 SIG. My guess is the higher cost of feeding results in some Buyers changing to a different caliber that is less expensive/more readily available?)