Tammi,
The plastic Class 15s are a bit smaller than the metal ones, at least the ones I have are. I don't care for them.
I "think" Sew-Classics recommendation of the plastic ones is because the new metal ones don't have the correct notch in the hub for the winders with the little peg on them. The winders with the spring work OK with them.
I think that's the reason. But I won't sign my name to it.
Joe