Either gun will be fine. I bought the Gen 4 for the bigger mag release and ambidextrous at that, 3 magazines and the smaller back for size adjustment on the grip. Those are the main differences between the 2. Which ever caliber you choose, either is fine. I have guns in both if you have shot the calibers before, then you know the difference. If not the 40 will snap back harder than the 9mm. If your recoil sensitive then get the 9mm, if not get the 40. Both are fine calibers.
22C Gen 2, 17 Gen 2, 26 Gen 4, 30 SF
40 Club number #164