The 21 is a brick of a gun to carry, but fine for shooting if it fits your hand (a big "if").
.45ACP is considerably more expensive than 9mm, if that's a consideration for you. .45ACP kicks harder. There are also some "interchange-ability" advantages in owning a 17 and a 26. I have both a 17 and a 26 and am quite pleased with the "matched pair."