I shot both the gen 3 and the gen 5 before buying the gen 5. Both guns shoot well and to some extent (as always lol!) it will be personal preference. For me, the gen 5 shot flatter and just felt better balanced, which might be partly due to the grip. The gen 3 grip was ok, but the finger grooves just don’t align with my actual fingers, so it interfered with how I gripped the pistol. The gen 5 thankfully doesn’t have the finger grooves. As others have said, the trigger starts better on the gen 5, and I didn’t plan to replace the trigger anytime soon. Also, I figured I might throw a red dot on at some point so the MOS is a nice feature. I also used my Glock certificate and the cost to upgrade, for me, was worth it. Added Talon grips and swapped out for Dawson sights. Good to go.
Good luck with your decision. Really, you can’t go “wrong”, per se, with either one!