-TRG-22 has "ears" on the mag bottom to help to grip it and pull it out. So stock fore-end is in front of the triggerguard radiused to accept these. TRG-21 mags have a slightly longer "floorplate" instead. So to use 22 mags in a 21, you have to grind the ears off, or mill out the plastic forend a bit.
-TRG-21 uses slightly different cheek/height spacers than the 22. Probably the same size/length, but the profile on the underside where it engages the "bumps" on the stock are different. 22 Length spacers work for both
-TRG-21 is probably blued verses parkerized for the bolt handle.
I would think accuracy is the same. But 22 has minor tweaks (mainly the mag ears) from testing/user feedback.....so I guess that is a pro, plus mags, etc more readily available. Same great trigger!