The 12 bolt isn't much beefier than the 8.5. The best thing about a 9" to me is parts availability. It is by far the most popular rear and they make everyhting for it. Also the fact that all Ford 9's have bolt in axles and no c clips. You can add c-clip eliminators to a 8.5 or 12 bolt but thay almost always leak and you end up with a smaller inferior bearing(at least the ones I have had were set up that way, but I haven't seen every kit available I am sure) The best way to get rid of the c-clips is to put on Ford ends and by the time you buy the ends and have them installed(really needs to be done in a jig) you can buy a bolt in 9" housing. They sell for around $350 for one that bolts into a G body.
12 bolts aren't much weaker then 9" and take less HP. The only reason I like the ford 9" is the ease to work on it. Pop it out and do your setup on the bench instead of under the car. Also it's fairly quick and easy to do a gear swap if you have another center section.