I think you'd be happy with either one. Both are good calibers. I've never owned either, but I've shot my uncle's Glock 20 (10mm). It had recoil similar to my 22 (.40 cal). His buddy had a Sig .45, that was a really nice pistol. But it definitely had more snap to it than the 10mm. That might have had to do with how hot his handloads were... I'm not sure. The balance of the Sig was also different than the Glock's. And since I'm more used to shooting Glocks, maybe that threw me off a little too.
Is there a gun range in your area that'll let you rent guns? The best way to compare is to select ammo that's closest to what you'd use in either pistol and then shoot both weapons back to back.
Both the Glock 20 and the Glock 21 are fine weapons... fine calibers. But I think you will find a recoil difference. I'd shoot both before making a decision. Good luck & happy shooting! :thumbsup:
I shot with glock 20 and 22 a while ago and I never liked them and the strike fire safety is not what I like. I prefer the para ord p16-40 which was the pistol that gave me the best groups at 50 yards, 2 inches and half groups. For what concerns the 10mm auto, the best gun I have shot in this caliber was a custom made high capacity 1911 caspian which belongs to my oncle, it had a kart barrel, specific adjustable sights, specific custom made ambidextrous safety and gi style push rod. That pistols shot near 2 inches groups at 50 yards. Amazing accuracy and very smooth trigger. I have also shot 1911 colt delta elites but those were pieces of crap compared to the caspian. The frames used to crack after a hundreds of rounds with highly loaded cartridges.