I have a Brother too and use only plastic bobbins. I just read this on another quilt board:
If your machine came with plastic bobbins, that's what you should stick with. The metal class 15s are intended for use with either top- or front-loading vintage mechanical machines and newer front-loading machines with metal bobbin cases.
Your machine probably has a magnetic hook, and the metal bobbin will interfere with its performance.
Also, just my guess, but the cover over the bobbin is probably see-through plastic, which allows you to see how low your bobbin thread is. Using a metal bobbin makes this more difficult.