Like you I have a vintage 1950 Singer that takes either. I think the metal is better and I use that most of the time but sometimes it is convenient to grab a plastic bobbin already filled.

Many of the newer machines with bobbin sensors come with plastic bobbins because the sensor can't read through the metal. That's not to say the new Bernina's with the huge bobbins like my new 770QE don't have metal in them, just different technology I suppose. Face it, we want to piece our quilt and quilt it without running out of bobbin or having to wind 10 bobbins to get the job done.