Good Day opd743 and et al.,
This is not to challenge the information provided by previous posts, I only raise some points of information that may assist you in your decision. There are a number of threads regarding this topic over on Glocktalk forums if you care to have a go at the information.
Essentially, there are several significant items of interest. First there is a pretty significant difference in the mass of the slides between the 20 (10mm) and 21 (45 acp). Additionally, the breachface of the 45acp is different from the 10mm.
I happen to own a 20 and 21 so I can confirm these points that have been brought up in the GT discussions.
Now with that said, I do use a conversion barrel with my G20 to run between configurations for 10mm and 40SW. That conversion is much more straight forward and quite effective, and cost effective too.
Best of luck in your information search.
Cheers!