I've handled and fired the mdl 21 sf, and owned a 21 gen3. As far as the feel of the gun goes, I couldn't tell a nickel's worth of difference in my hand. It may feel different to you. Whether that's better or not is for you to decide. The recoil system in the gen 4 is IMO change for the sake of change. I have yet to find, nor have I heard of an advantage over the earlier system. Seems a lot of people are spending money to convert their gen 4s back to the gen 3 recoil system. I have quite a bit of experience with the mdl 20 gen 3, and like it a lot.
If the gen 3 were the same price as the gen 4, I'd still buy the gen 3.I'd bet the mdl 20 will handle all the full power loads you'd want to put through it. With the +2 extensions you have 18 rds of 10mm in the gun. That should handle about anything on this continent.
You might even consider the mdl 21. With a simple barrel and mag change you could have a pistol chambered in .45acp, 10mm, 357 Sig, .40 S&W, or 9x23. With some spring and buffer changes there's a few more like the .460 Rowlan/ .400 Corbon, and likely a few more I'm not familar with. How's that for versatility? Sights? I'd go for a Dawson adjustable plain black rear and the .115 wide FO. For me it's the best of both worlds in speed and precision.
Savage