Sorry to hear about your Gen 5 21s functioning problems. Ammo is definitely too expensive to vet them even if you sent them back to Glock. That problem and fix will eventually come to light, but that time has not yet come. Don't blame you one bit for selling them. It would be hard to put one's faith in either one of them as a self-defense pistol
I have a Glock 21SF gen 3. The only complaint I have about it is the slippery grip. I know I could put tape on the grip but I went a different direction. I'm waiting for my 21 gen 4 to hit my FFL Monday. I have a gen 4 20 in 10mm. I like the improved grip texture and larger mag release button. For me, the gen 4 20 with no backstrap added feels slightly better in the hand than the 21SF. From what I've read, the gen 4 20/21 with no added backstrap supposedly has the same grip dimensions as the 21SF. I just prefer the gen 4 to the 21SF basically for the improved grip texture and larger magazine button. If you can, check out the grip on the 21SF before buying. Also, as a point of reference, I like the grip texture on the M&P 2.0 as well. My M&P 1.0 has a slippery grip texture which was fixed by the 2.0.