Both Gen 3 and Gen 4 Glocks are readily available from most gun dealers. In the end, this isn’t all that different from buying a car, and choosing between this year’s model or the 3-year-old model.All things aside, the newer model tends to have significant improvements over previous years. Same with the Generation 4 Glocks.
As often happens with a broad new product release, there were some issues with the initial production units, which were rectified by Glock. But aside from that, I’ve not seen any substantial reasons to opt for the old model, aside from saving the $40 price difference.
Here’s a great run-down from TheYankeeMarshall on the differences between the Generation 3 and Generation 4 Glocks: