|: Unless you are very heavy, plastic trucks work better on streets then metal. Metal rattles.
I've never had metal rattle...ever. Keep your bushings in good condition, and replace them when needed, and you'll never have a problem. Plastic will warp and flex much faster than metal. But, for light skating, plastic will be the better 'light weight' choice!
It all depends on what type of skating that you'll be doing. Most of the time it will not make a difference on the plates unless you are a speed skater.