Pretty sure Mile High is correct.
I've had both types and Eternabond on my '94 helped seal up leaks in the rubber roof, and I've used it on the end cap seams, and will use it along the sidewall seams of my '02 full fiberglass roof.
Thing is that both roof styles have problems and it takes some effort to stay on top of them (heh).
The rubber roofs shrink and tear or get cut. The fiberglass are extra thin and the edge seals or a cut out seal in the roof can fail and wind can get under the roof and rip it right off. I've seen pics here on the forum of fiberglass roofs that were nearly half gone because of wind.
Having had both, I do think the fiberglass is superior, but it's still a maintenance item. I still have the job of running 2" Eternabond along each edge to prevent the roof peeling off in heavy winds.
Here's my blog post about the end cap seam taping...
Roof taping...
