No sag in Monster Cages. The ceiling actually has braces in it and you can stand on top of them. I saw a thread here about an AP cage with possible sag. I was a bit surprised and am skeptical that is the case, but it was involving a RHP not fitting to the ceiling properly.
I'm looking at around $500 for the Monster so I'll likely visit him again. I've been to the facility once already.
I do like the AP look though, however my Pro-Lines are pretty unbeatable as well.