If they fit, it doesn't matter.

YES! The brother bobbins have a groove inside the center, and are taller. Read the page about winding in the instruction book!
They gitcha!......and me too

Don't throw them out!
Use them this way. Wind thread on them and carry them for handsewing. Several of them will hold a lot more thread than a lumpy unhandy shaped spool.
Bunches of the filled metal bobbins fit into a little pocket kit.
I also like to wind decorative thread on them.

I was told by my dealer that the metal ones will ruin your machine. I took my metal ones to them and they traded with me. It didn't matter because I wasn't going to take a chance. After all purchasing new bobbins are a whole lot less expensive than buying a new sewing machine.

I have a Brother PE700II embroidery machine that came with plastic bobbins. Constantly having problems with tension etc so I decided to try metal bobbins. Sewed perfectly and have not had a problem since and it has been a couple years. I will stay with the metal and save my sanity.

I've never had problems using the type of bobbins that came with a particular machine.

I have had problems using plastic bobbins with a machine that originally had metal bobbins.

Don't recall whether I've used metal bobbins on a machine that came with plastic ones or not.

If it came with plastic you should stick with plastic.

If one bought a used machine, how would one know which type to use?

My machines are all older, and the instruction books (for some of the machines, anyway) didn't even bother to mention what type of bobbin(such as 15, 66, whatever) to use - much less whether plastic or metal was more appropriate. As far as that goes, some of the machines may be "pre-plastic bobbins"

I sure wish bobbins had some sort of imprint on them stating what they are.

Don't use metal if your machine calls for plastic. You can use plastic for metal tho. The metal wears the bobbin case.

Check the height and width by putting them on top of each other, then side by side. When servicing machines, I use metal bobbins whenever I can. However, some machines will work better with the plastic bobbins. If they're the same size, sew a zig-zag stitch with the plastic, then with the metal and see if there's a difference. Plastic bobbins do get rough edges, and need to be thrown away and replaced.