The plastic fenders tend to crack, nick, scuff and otherwise become dingy and gross over time, and aren't really lighter than alloy fenders. Metal fenders can be cleaned and polished, and look good with a little "beacausage" denting here or there. Some people complain about the "rattle-ting!" of pebbles sucked up into a metal fender, where I don't like the "klickety-kluck" of plastic, but that's just picking nits.
In terms of aesthetics and longevity, I gotta give the nod to metal. (I actually have stainless steel fenders on my ride... no weight weenie I!)