I am changing my front door from a wooden one to a plastic one. It seems I have two choices - GRP or PVCu. Except for the fact that the GRP one can be coloured, what are the differences between the two types? The PVCu salesman says that GRP doors warp with time. The GRP salesman says GRP doors are much more robust and are gradually replacing PVCu doors. And I'm not sure that the salesman who sells both isn't trying to sell me the one he makes most commission on. Any advice would be welcome.