Currently, the latest model is our M3D Gen 4 which is based on the Glock 19 Gen 4. The M3D Gen 3 is based on the Glock 19 Gen 3 and is a special-order product (please contact us if you need a Gen 3). However, our conversion service is available to both Gen 3 and Gen 4.
From a cosmetic standpoint, the only difference is the grip texture. The Gen 3 is smooth, while the Gen 4 has a checkering pattern that helps with grip when your hands get sweaty. Internally the recoil spring on the Gen 3 is different versus the Gen 4.
If you are purchasing a new M3D from us we recommend the Gen 4 (the current model), because of the improved recoil spring and textured grip.