I have only used plastic. Is there an advantage to using metal ones?

The rule of thumb I've been told is metal for metal bobbin cases and plastic for plastic ones.

I use the same type that came with each of my machines. I have been told if you use a metal bobbin in a machine that came with plastic...you will wear everything out faster as the machine was never meant for metal bobbins and visa-versa.

I sew with a vintage machine and I prefer metal over plastic. Over time the plastic wears and distorts.

Plastic can get burrs on them too, which can make your stitches mess up. I soooo prefer the metal over the plastic, but my new machines can't use them :D:D:D

I believe you can fill the metal bobbins with more thread than the plastic ones. Have you noticed this or am I confused?

Think you might be right .... seems that way to me anyway.

I use either.

Please use whichever type of bobbin that originally came with your machine....if it is plastic...stick with it...even if the metal fits.
You can cause damage to your bobbin case with the wrong kind.

Yes!! Vintage machines and machines with metal cases were engineered and manufactured to take metal bobbins. Newer machines with plastic bobbin cases are engineered and spec'd for plastic. Just because it fits, doesn't mean you should use it.