.357SIG is more expensive (and some say less comfortable) to shoot for what is likely a negligible gain in "stopping power" over 9mm+P or +P+. .40S&W will certainly do most anything a .357SIG will do.
If you like the Glock better than the XD - and I do - why not trade? Defense guns are just tools. Choose and use the tools that work the best for you.
Alas, I have no first-hand experience with the 21SF. I've been deployed since it was introduced.