i used to use the thor plastic chest protector. it was good for keeping the mud off my shirt and if i was sliding along the ground on my front. i used it for abour 2 or 3 years of racing mx. at the end of last season my friend stopped his mountain biking all together to start racing mx all the time. he used a "dainese" body suit for the down hill events he went to, i got to try the suit when he didnt need it. the difference is night and day between the 2, the suit was alot more comfy and keeps you hella warm on cold days, and when i was sliding along the ground on my front after coming off, i didnt feel every bump i was hitting on the ground. also, there is a bit more freedome of movement.
id recomend you get a body suit of some sort. IMO, the plastic armour is flimsy and with a big hit would break. with chest, elbow, back and kidney protection in one suit i dont think you could go wrong.
then again, depends what riding your doing?