Both are a great choice, depends on which grip texture you like. The Gen 4 is a little more in price, but it comes with 3 mags. Has the adjustable back strap to help the fit.I prefer the feel of the Gen 3 myself.
I ordered a 4th gen since i have two third gen glocks. figured for the same price why not try a slightly new design, with interchangeable backstraps and a more durbale recoil spring ive heard nothing but good things about them