Chris Bosh . . . is much, much better defensively than Big Al.
Chris Bosh's defense isn't much, much better than anybody's, including Spud Webb's.

Big Al's defense is perhaps being maligned a bit unfairly, based solely upon reputation. Last year, he was actually fairly respectable there. He was top-50 league wide in both post-up and P&R defense, and his points per possession was 70th overall. That's very good. Even acknowledging that stats can be misleading, it's unlikely that Big Al would put up defensive numbers like that if he was abjectly terrible. He's probably about average as a man-to-man defensive center, with the numbers saying he's above-average.
(For comparison sake, Bosh was 149th in the NBA on post-ups. There's a reason Shaq called him the Ru-Paul of big men; he can't defend bigger players down low, period.)