What advantages (besides price) is he touting?
IF you need strength then FRP has big advantages.
Either way the adhesion is the primary issue, make sure that you know what combination of surface prep and base coats are being used to assure proper adhesion.
The coatings aren't even close to being equivalent. Which one do you need? I have no idea, but I'm pretty sure there is some API and/or NACE guidance available on the topic if you do some research.
I would not use a polyurethane in this situation. I would recommend a Cyclo-Aliphatic Epoxy. They're designed for immersion in these situations. Polyurethanes are generally used for UV protection, gloss retention, etc.