Welcome to TFP! I am in the process of getting permits for a fiberglass pool. I have done a ton of research on different manufacturers, the installation etc. As far as warranties, all of the companies pretty much offer a lifetime guarantee on the shells. However, typically this only means the structure is guaranteed against leaks. Make sure you read the warranty in it's entirety! The surface is another area of the warranty you want to look at. I have chosen Viking. I know they cover the surface regarding osmotic blistering. Most other complaints regarding the surface has to do with fading (this is often after many many years). Like a corvette, a fiberglass pool is fiberglass. The same polish you would use to restore the shine on a vet you can use on the pool. This is usually only around the tile line area (if there is no tile). This is another reason why you should never empty the pool. Once you have checked out the warranty and you are satisfied, the most important thing (even more so than the warranty) is the PB.
Make sure you get a reputable PB and get everything in writing. My PB specializes in fiberglass installs. If you can find a PB that only, or mainly does fiberglass installs I highly recommend it. I have been invited to the installation of another customer so I could actually see the process. My PB is very good at what they do, and are so confident they allow customers to observe their work prior to purchasing. I don't care how great the fiberglass manufacturer is, a shoddy installation ruins the whole deal.
Besides that, go with a manufacturer that has a shape and size that you like. That's pretty much it. I recommend you stay here a while and read up on balancing your pool as I know at least Vikings warranty is voided regarding issues caused by unbalanced water.