To answer Joe's question, I do not have polyurea in my pond yet, because the pond was built 5 years ago, and I just got in to polyurea 1 year ago. I used a two part component which I purchased from a local koi dealer. It is kind of like CIM & Sanitread. I have been having problems with it peeling off the bottom and sides of my pond, and therefore I will need to drain, sandblast and recoat with polyurea.
With the high demand of polyurea right now, we have sprayed aprroximately 40+ ponds in the last year, and therefore my project has been put on the back burner. What got me in to polyurea in the first place was it's potential to stretch with the movement of the earth, and it's adheasion factor. A product which I feel stands alone in comparison with epoxy and 2 part applications with no tensil strenghts for movement within the structure.
It goes without saying that there is 2 types of concrete, one that can crack, and one that will crack. I don't know much about fiberglass, but have seen surfboards that have been dropped, have seen boats that have been bumped, and have seen many cracks on fiberglass. I actually coated a fiberglass pond a couple of months back, because of the cracking problem. The pond was about 20 years old, sunk in to the earth, and all the bulkheads were rotted around the penatrations. I had to drill holes all over in the shell to get adheasion as I did not want to rely on the super smooth surface of fiberglass. As a matter of fact, there was not much alge on the sides and bottom

! I would imagine if you were able to put a gel coat on both sides of the fiberglass, they probably could have eliminated the rotting of the resin and glass that was exposed to moisture.
DISCLAIMER!!! I KNOW NOTHING ABOUT FIBERGLASS NOR DO I CHOOSE TO!
My opinion, as far as pricing is considered, there is no comparison. You are comparing apples to oranges. Polyurea can be applied straight on to geotextile as a membrane, which would eliminate the cost of gunite, rebar, and labor to put on coating. As far as pinholes are concerned, Ruben's statement is correct. If a concrete shell has too much moisture behind it or in it, the heat of 170 degree polyurea will pull the moisture to the surface, creating pinholes. To eliminate that you have allow for a full cure for at least 60 days, and put several coats of primer. This I do know for sure by experience.
Joe, you got me hook line and sinker....
Jeff Dunkel