Remember that recoil is only a portion of the equation. As I've said often enough here, my wife started on a 20ga and has since moved to a 12ga. For her, I don't think recoil was the biggest issue. Her main criteria was that the gun be light enough for her to easily handle. Therefore, shooting a 20ga for a couple years helped her build up some shooting-specific muscles and get comfortable.
I've seen the same thing teaching some younger shooters. Bigger, heavier guns may have less recoil... but they can also be more combersome to use. For some new shooters, they may have a preset idea that 12ga="too big". You can then use a 20ga and let them psych themselves into being comfortable with a "little gun". Then, when they later try a 12ga, they are likely to think "Hey! That's not bad."
Either way is fine, and as long as you stick with light target loads in either gauge, the recoil should not be too much of a problem. IMO, if you think you may be training younger/smaller shooters, then a 20ga should be seriously considered. If not, the 12ga is probably the way to go.