I guess it depends on how "factory" you want the finished product to appear, and how bad of shape the original skin with its built-in reinforcement is.
If you want a "factory" look, the fiberglass won't do because its missing the channel to mount the trim strip. Usually they also look a little "twisted" at the ends, but careful reinforcement would probably cure that.
If the original urethane skin is buckled from its cast-in reinforcement plate corroding, you'll have a tough time ever fixing it, but I recall that someone here tried. If you have a crack or crater in the exterior, then the urethane repair products from 3M might fix it right up.
For a custom front end (i.e. modified header panel, modified fenders, or attaching an air dam to the bumper), the fiberglass bumper is the way to go, especially with the aluminum reinforcement you are considering.
I have one decent set of bumpers (out of 3 MII's!) that I am fixing up for my Ghia. I removed the reinforcement panels, had them sandblasted, and painted them with self-etching primer and Eastwoods Detail Gray paint. I'm about to carry the bumper skins to the sand-blaster so that he can clean the exposed part of the cast-in reinforcement. I plan to bolt them to some 2x4 scrap so that he won't muck up the exterior of the skins dragging them around his shop. When the reinforcements are done and bolted back on, I'll carry them to a body shop for final finishing and painting.
They are made out of "unobtanium" afterall...