I had a GRP tank in a previous boat that proved trouble free for 10 years and as far as I know is still performing well. One thing I did like was the ability to be able to actually see the fuel level through the side of the tank.
I used to be undecided but now I'm not so sure.
Beauty is in the eye of the beerholder!
If you go with GRP be sure to use the correct resin. Standard polyester will not do, there are some vinyl ester resins that will last even with biodiesel or gas with ethanol. I will be building GRP diesel tanks for my project in the near future and have contacted the resin suppliers for resin specs. Like anything else, workmanship counts if you don't want the tank to shed from the inside out. GRP tanks don't corrode, which to me is a big plus.
Chuck Paine Sarah 32 under construction
I am replacing the SS tanks both water and fuel with epoxy GRP tanks built into the boat. From my research bio diesel should not be a problem. The problem is ethanol added into petrol, if you are using petrol as a fuel you will probably need to go to aluminum.