said by whizkid3:
The benefit of a metal box is that should a hot wire come into contact with the box, it will short-circuit to ground and trip the circuit breaker, de-energizing the circuit, rather than posing a risk of shock or fire.
Plastic boxes generally are for type NM wire only (Romex), meaning that in most localities they can only be used in small residential work (1 & 2 family homes).
Although more expensive, I find metal boxes easier to work with. As far as extra work grounding, etc - that is not the case if you work with type-AC or type-MC cable. Its actually less work to use all metal and self-grounding receptacles. There is virtually no grounding work at all.
That being said, I prefer metal boxes in all my own work.
Metal boxes will oxygen deprive any fire inside them (assuming you have metal faceplate), which is why all commercial and high rise buildings use metal. Metal boxes and MC/AC cable stop oxygen from getting and smoke getting out of the electrical system, especially important in office buildings with suspended ceilings that act as return air ducts. Even with plastic faceplates, fire shooting out the front of the box is better than fire shooting up behind the wall which will take more time to recognize. And the advantage of shorting to ground of metal box and AC/MC cable is a huge safety feature in my opinion. Cockroachs and termites can go inside boxes and start problems. Insects love warm wirenuts. Much better and safer than the AFCIs the industry is now pushing. Only problem with metal electrical systems is extension cords. They rarely have a metal shielded/braid/jacket that a hot wire could arc to and trip the breaker, except for the "fire guard" AFCI extension cords.
AC/MC also reduces undetected "nail through the romex" problems. Someone has their siding replaced, the romex gets turned into swiss cheese from the nails