As a test and development engineer (the hands on breaking stuff kind, not the desk jockey kind) I don’t trust anything Sig makes until I’ve proven that sample for myself. Strike one.
I had a couple of the 320 pistols, and really liked the fit and feel of the compact model. Just a nice grip shape. The stock trigger wasn’t great (neither is a Glock), but the Greyguns upgrade felt pretty good. The nice feel didn’t translate to better results on target though; probably because I have more time with Glocks at speed, but even in slow fire the 320s didn’t give me any advantage.
But the POI was low compared to any other pistol. Sig loves that stupid “combat hold” where the sights cover the target. I see no good reason to have a pistol that shoots lower than all my others, but the dumb sights for them were around $100, and it was questionable if I could get POI high enough anyway. Strike two.
Another minor quibble was the ballistics. Maybe specific to my sample, but the compact model (slightly bigger than a G19) ran the same ammo about 100 fps slower than any of my Glocks if similar barrel length. Not a big deal, but why give up something for nothing? Strike three.
Accuracy was a wash with my Glocks, no winners there except the better aftermarket Glock barrels, but that’s not a fair comparison. Sig made the 320 barrel diameter smaller than Glock did, leaving very little room for a shoulder for threaded barrel suppressor use. Point to Glock. Simplicity of the internals - point to Glock.
Overall, while my 320 pistols weren’t trash, I struggled to find anything to commend them over a Glock 19 except the feel in the hand. Not being a Sig fanboy, I saw no reason to keep them.
Personally for me, if I were to switch to a different striker fired pistol from Glock for better feel and accuracy, it’d be to my Canik TP9SFx, or some smaller version of same. Accuracy is better than any of my Glocks or 320s, and the trigger (with very minor improvements) is better than any other striker pistol I’ve tried. But I’m not convinced it’s as safe for carry, therefore a Glock 19 is in my belt.
And FWIW, Gen 4 or 5, either way depending what you think about finger grooves. The internals are different, but not in a way that functionally matters IME. Take your pick.