My first Glock was a 21. I was surprised by the recoil from the 22. 21 shoots smoother and less snappy than the 22 IMO. Try one out first and tell me what you think.
After being on this Forum for awhile the 30 seems pretty appealing and you can run 21 mags out of it.
Why not pick up a G30 for an EDC/CCW version of your G21. That way you can use the same ammo and even use the 13 round G21 mags in the G30 for around the house or at the range? Just figure if you already have a G21 in 45acp and like it then the Sub-Compact would be the next logical gun to get.
If you want a larger size, I'd say go with a 37 or a 17. I really liked the 37 gap when I got a chance to shoot my friends. Personally, I really do not like the .40 round. But you also can't go wrong with the 19.