Plastic is a much better choice
Fiberglass weighs considerably more, and anything that can happen to fiberglass anywhere else in a boat--blisters, cracks, rot etc--can happen to a fiberglass tank. Since it almost always starts inside the tank it's not likely to be noticed until enough damage has occurred to make it noticeable outside the tank. Building a fiberglass tank will cost about the same--considerably more if you consider your time to be worth anything--as a good quality plastic tank. The only upside I know of is that a crack or other leak in a fiberglass can be repaired, polyethylene cannot.No matter what the tank is made of, it won't prevent or cure odor inside the boat OR out the tank vent line.