Both players have different warts offensively, but both have the same warts on defense that get exposed in the playoffs.
Parker is the better point guard and better facilitator, and he allows Kawhi to take a damn breath here and there. Mills is the opposite, he's not a facilitator -- but he's a better spot up shooter than Parker. Both have different strengths, but each of their strengths are inconsistent from a game to game basis. Mills isn't money every game from the perimeter, he's actually off more often than he's on. When he's on, he's on and he'll go 4 for 6, but he'll be off the next game, two, or three or four.
Utilizing Mills more at PG, could mean a step back from Kawhi from an efficiency or defensive standpoint because of the the type of stamina that's exerted in the playoffs. He will need some playmaking help offensively in order for him to sustain the type of defense that's needed to win games -- Mills can't provide that.
All in all, both players will consistently be abused as pinatas defensively in the playoffs, while being inconsistent from game to game with their strengths on offense... its inevitable. We've seen it the last two playoff runs. And it's hard to choose one or the other, both have major flaws and both have skill sets on the offensive end that help Kawhi in different ways.