Bynum's more polished in the post but that doesn't even make him a better offensive player than Howard. He's not as strong or explosive and he can't run the floor. Bynum's numbers are similar to Howard's but Howard faces double teams on a regular basis.
Then there's the whole defense thing...
The fact that Howard facing double teams is not relevant - Bynum gets double/triple teammed just as much. If you were the opposing coach, would you want either 280+ lbs big to bully your team?
I don't think there should be any doubt as to who gets more of the opposing teams' attention. Howard gets more, a lot more, because he's the best player on his team and the best offensive weapon.
Howard gets more touches because he's the star player. If Bynum was the star player, he'd go to the line more, and thus, shoot more free throws at the rate +20% better rate than Howard. That's my reasoning. You are not going to find a center as dominant as Bynum who can make his free throws (e.g Shaq).
That's a possible scenario, but its not the current reality. All we have here is the actual reality, and that is that Howard and Bynum shoot from the field at nearly the same %, despite Howard facing more resistance.
Interesting note though: per 36 minutes, their #'s are almost identical from the field. 12.7 FGA's at 58%. The biggest difference is that Howard goes to the line more, but he makes less of those attempts.
Larry Bird was not exactly the most explosive player either, yet look at how he dominated. Bynum gets the job done inside just as well as Howard. If dunking ability is what makes a player great, than Blake Griffin is better than Larry Bird.
Dunking ability doesn't in and of its self make a player great, his ability to help his team win makes a player great, and right now Dwight Howard does more for the Magic than Bynum does for the Lakers.
Athleticism is not relevant if both players are near 7 feet tall, weight 280+ lbs, and make everyone else look like Kevin Hart. The post game, though, is what separates Hakeem Olajuwon and Kevin McHale from Hasheem Thabeet and Cole Aldrich.
Come again? Athleticism is not relevant because they're big? Athleticism in a big man is the difference between Scott Pollard and Shaquille O'Neal.
Plenty of guys have size. A very small fraction of those people have the athleticism to be 'slow' in the NBA. An even smaller fraction of those guys have the elite athleticism and size to be gamechangers, ala Shaq, Hakeem, etc...
Athleticism absolutely does matter. Howard is more athletic, Bynum is a little more skilled in his post game. They're both elite centers, but Howard is the better overall player if you look at both ends of the floor. Offensively, because of Howard's greater odds of drawing fouls (which isn't just about making FT's but is also about getting the other team in the penalty faster), and his vastly superior play on the defensive end, he's the better player.