I happened to read halfords policy on carbon fibre, it was that the guarantee was minimal, alloy fairs better with steel having the best guarantee length for parts, now I don't want to kill the industry, but they said that after one crash, carbon should be checked or ditched as the layers of carbon split, that doesn't sound like good mtb philosophy, unless of course, your planning to use it on road alone, Ive never bought carbon for that reason.
alluminium used to crack under stress, its getting better, while steel bends,
my best advice is use a cromo fork if you want rigid, stunt planes use it.
however Ti may be cool.
which is more enviromentaly friendly, I have no idea?