I have both, I perfer plastic - for the same reasons listed in above posts . . . .
One thing I do is leave my bolts/nuts/ washers in the bags I get them in. Makes it easier to keep seperate if somehing gets tossed into the bin, and to keep the bins cleaner. The bags I get the fastener hardware in are ziplock - many times I'm working out in the elements with them, so the ziplocks really help with keeping the bin clean and hardware from getting wet/debris laden.