I would debate that Marvin is better on D. Marvin stays in front of his man better because he doesn't put as much on the ball pressure. Carroll gets right up there on his man, which is why he creates so many turnovers. Marvin does have the ability and length to be a good defender but he lacks the intensity that Carroll brings. Do you ever recall Marvin putting full court pressure on his man?
Marvin's overall problem is lack of intensity and focus, and why he hasn't and probably never will live up to his potential. I had a friend whose son was like that -- 6-6, could jump through the ceiling, could put his elbows over the rim. Got a basketball scholarship and quit the team freshman year. He just didn't have the drive despite all that ability. You need both in the NBA. A little ability and a lot of drive will go further than a lot of ability and little drive in pro sports. Think of Pete Rose; he's a good example of the former. And it might said to apply in most professions as well.