My guess is that since you're playing against a backgame, you would prefer his front checkers crunch, and hitting the checker on the 15 point means that his front group of checkers are more forward on average. If you hit the checker on the 21 point, you're giving him more time to escape another checker and keep himself from crunching.
And from that logic, I would guess that 16-15 rates as the slightly better play. I probably would have played 24/22 8/7 and avoided hitting completely, but I have no idea if that's better.