I encourage you to go to a shop and hold a G21. I have one and I really like it but the grip is about as big as any gun I've ever held. My hands are large enough that it's not a problem at all but others have trouble holding it. Too look at it, the SF appears identical but when you hold it you will feel the difference.
All of my large frame Glocks have been in the SF frame. The standard large frame Glocks don't feel good to me, but the SF frames fit my hands (medium glove sized) pretty well.
I own a 21 gen3. with two hands its not a problem. a single handed quick mag dump is a challenge. If you can find a SF for a decent price I'd get it. I got standard 21 in a trade.