What is most important is that you use the bobbin that are recommended for yiur machine! They are recommended for a reason by the manufacture. I have a Singer that takes 15J bobbins not the 15. The 15 will work in the bobbin case but they are a little loose. 15J are more expensive but that is what Singers recommends. My Brothers uses a specific metal bobbin that is not avaliable in plastic but I have never tried to use a different number either plastic or metal. I have used plastic in my singer and really do not see much difference between the metal vs plastic as far as durability.