I'd absolutely get the cheapest fiberglass you can. Why waste funds on something that has no demonstrable improvement? By demonstrable, I mean laboratory data in the low frequencies. Cheap fiberglass has the edge.
You'll find a lot of guys here on AVS that have done exhaustive door systems and many that have a single door only. If the area outside the single door has a stairwell headed up, or major ductwork overhead, then you may be dissatisfied with a single door. You can also look to beef up that single door, or look at dual steel doors (low$$). All in the Door SIM.
The door can certainly limit performance, but impossible to say by how much as there are innumerable variables.
I have been tempted to write an article for guys with french door wives. Seriously, it's that common. So no worries.
At the end of the day the door is rated based on its mass and seals. Get the heavier door.