I think MPJ has been a good (above average) defender in the past, but last year he had some shoulder issues that really impacted both his offense and defense. I don't think last year is the best one to consider for his future defensive value, he was obviously hobbled but kept playing due to necessity. I would think after a full off-season he will come back into form. But I would agree that last year he was pretty bad on D.
Cam is not a good defender though. Now whether that is because he was on a shitty team with the Nets, had a bad coach, or just didn't care about defense, I don't know. Certainly there is room for him to grow, and a huge incentive for him to play hard both ways in Denver, but I don't think it's going to have a huge impact. He will the the third/fourth option on this offense and will need to make his money via sharp shooting and being a team player.
MPJ when healthy is a better defender overall, maybe only by a few points per possession. However, swapping them and getting Cam who is about ~6% better at hitting 3s may end up being a huge win for Denver.