metal give a harsh/bright "twangy" sound
in general, thicker plastic gives mellower tone and quicker/more correct response. thinner picks give thinner sound.
I use a Blue chip thumb pick, which is rock solid ( part that hits the string won't bend) and has tremendous response form the strings. i like the sound of some plastic picks picks too (clown barf in particular), but I prefer the part that wraps around my thumb to be metal as I seem to be able to shape it for comfort better, and it stays there and doesn't break like plastic can.
It really comes down to personal taste. Experiment with a few and see what YOU like best.