I like fiberglass... Ran m&m and gman plastics in the past, they broke real easy especially in cold weather. Fiberglass can be fixed (self taught pro at repairs!). A little sanding and work you can do wonders, re-enforce where you need it. I fixed an ultramax body on my kart back In May (the one in my avitar picture), re did the whole area connecting the floorpan to the nose, finally started developing a hole the last week out, and I ran into a lot of stuff hard, body survived 4 seasons on dirt bullring and indoors. Also repaired a shadow nose that was 5 years old... Floorpan was in 3 pieces hanging on to the rest of the nose by threads just on one side, solid as a rock now, have before and after pics of all my projects If your interested in seeing them...
The one benefit of gman plastic is they are light, a few buddies I know that was 230#+ try them to skimp on weight, like I said they do not hold up though