I went with the 40 cuz I was MUCH more accurate with it (beyond 40 yards). The longer barrel and slide, DeltaPoint optic with co-witnessed Hawkins suppressor iridium sights is quite nice. Shoots like .40 caliber G22, nice balance. Tried the G20 with the longer KKM barrel but my groups were not as tight.
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Holster transfers from pack belt to pants in a second for the final stalk.
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The G40 MOS replaced my S&W 329PD.
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Like you, my eyes are not as good as they used to be and I think the red dot helps, especially in lower light. I have done SD training with the G40 so I could add it to my CCW permit and drawing it and firing in under 2 seconds is no prob which surprised me. When I travel I hate having to take two sidearms (one for CCW and one for backcountry sidearm). I can take the G40 and swap out the magazine from hard cast to SD ammo in a flash. Works for me but everyone is different. Best to shoot em first, then buy. BTW...I'm a big boy with large hands which may be a factor. The Glock 40 gets a lot of envious looks here in the People's Republik of Commiefornistan. LOL
Do your homework...lots of experienced opinions out there.
https://www.glocktalk.com/threads/g20-vs-g40-for-hunting-and-woods-gun.1631792/