The G21's slide is milled out, making it 2-3 oz lighter than the G20, IIRC.
The G21 can be converted to 10mm, but the slide will be a bit lighter. For factory loads, no problem. For a handloader, the G20 can be loaded a bit hotter, using the same springs, as long as chamber support is not an issue. (If it is, then aftermarket barrels are available.)
The G20 can shoot 40SW. And you can get aftermarket barrels in 40SW/357SIG, and even 9x23.
Why limit yourself to one or the other? Get 'em both. You can even have two different frames gen3/gen3SF/gen3ambi and mix and match.
If you're not a handloader, the G21 is probably the way to go. There's nothing wrong with 45ACP, and the Rowland conversion looks pretty impressive.