I chose the Gen 3 because on the Gen 4 I would occasionally accidentally hit the mag release. The Gen 3 mag release is small enough I think it works better for the small frame of the 26. I'd recommend the Gen 3 then add the Vickers mag release if you want a little easier to reach mag release without causing an inadvertent mag change
The Gen 4 is more bang for your buck, IMHO. It gives you an extra mag, an extended mag release, a dual recoil spring, and interchangeable backstraps to help it fit your hand better. If you are gonna have just one, get the Gen 4.
That said, I like the Gen 3 better and is my EDC. The Gen 4 would pinch my hand between the mag and front strap while firing, the Gen 3 does not. Dunno why.
Some ranges have both Gen's for rent, so I would shoot both if possible. As usual, YMMV.
I have also experienced that the Gen 4 mag release will release while I'm shooting, with the way my hands grip the gun. Very disconcerting.
I vote for Gen 3, because I also like the fact you can tailor the length / amount of protrusion of the mag release by substituting the corresponding part from a different model Glock. They are interchangeable, but there are different lengths to account for the different frame widths. It is a cheap part, and you can buy a whole set of them and swap them out until you get it just right. On this particular point, I wish every gun was like that. For example, you can put the .45 one in a 9mm gun, and get an extended mag release for a couple bucks.
Im not a pistol guro. I never could get accustom to wearing 2 shirts, and the gen 4 texturing rubs me raw. I carried one for the first time last week, and am right back to my 3rd gen.