Cast iron will last you forever, as said already concrete ones split, crack, break, etc. Also, you can get more weight on a bar using cast iron, than vinyl/concrete. With cheap wide argos plates i can get just shy of 50kg on each of my dumbell bars, vinyl and i'd be looking at about half that.
Chrome is nice its just a b*stard to clean and its more expensive. Why get showy chrome plates? My dumbells aren't the centre piece of my room, they live in the cupboard/garage until needed, and i don't show them off to my friends at how beautiful they are...so chrome is a bit pointless, unless you've got money to burn.