If you are going to leave the gun slide stock, go with the Gen 4. i personally like the Gen3 single spring if modified with a Wolf spring. Also a few good spring guide rods rods out there. With a single spring it is easier to adjust the slide to work just right. I don’t want the spent brass falling out of the slide, or flying 10 feet in the air. I am told it’s just right when the spent casings are 2 to 3 1/2 feet from the gun. The ammo you decide to use will have an affect also. But stock to stock the Gen4 seems to be liked by more shooters. Felt recoil seems to be better with the Gen4 as well. I also like the Gen3 transfer bar better too. Some of the after market triggers come with a stock Glock Gen 3 transfer bar. It’s pull is about a 1/2 to 3/4 pound lighter. The Apex trigger kit comes with a stock Glock Gen3 transfer bar.
The New NDZ Generation 4 Stainless Steel Guide Rod Assemblies are designed specifically for Generation 4 pistols and allow a user not only to have a guide rod that will be more durable but also allows for different spring weights to be purchased or changed which means the ability to precisely tune the pistol for optimal performance. The NDZ Generation 4 Adapter Ring installs under the retention end screw thereby making up the difference of space in the front of a generation 4 slide as well as retaining the spring accordingly. Our Generation 4 specific guide rods have an enlarged base similar to that of the stock OEM guide rods allows for the single spring assembly to sit at a more parallel position to the barrel for better performance. You can choose from 11-22 pound springs for the stainless guide rod.