When I dive with a BP/W and crotch strap, I rarely find myself in the position of having to wear a weight belt as well, but when I do, I put the weight belt on first, and I do it because I want the crotch strap to interfere with the falling belt.
What?
I figure that it will take a really serious emergency to make me drop my weights, and that will be an extremely rare event. If it does happen, the extra couple of seconds required for me to free the weight belt from the crotch strap is not going to make a whole lot of difference. In short, it is extremely unlikely that I will need to do it, and the consequences of having it under the crotch strap aren't all that great.
On the other hand, it is very possible that the weight belt buckle can come open accidentally, especially if I am trying to work my way through close quarters. I had a weight belt buckle come open on a dive, fortunately at the very start of the dive, when I was a beginning diver, and those weights were never recovered. When I teach students the giant stride entry in the pool and one of them forgets to put the left hand on the buckle, the odds are that belt is coming off and dropping to the bottom of the pool. If my weight belt comes open accidentally, I like the idea that the crotch strap will keep it from falling off and sending me on an unplanned trip to the surface. In short, it is more likely that the weight belt will come open accidentally, and the consequences of losing those weights can be serious.
Thus, I play the odds and put it on first.