In BFD2 you can load your own samples (wav files). In SD2 you can only use the samples which belongs to the provided samplepack.
But BFD isn't really useable for metal style music unless you buy a samplepack with better samples for that kind of music..like "Joe Barresi Evil Drums". This addon exists for both programms, bfd and superior drummer. SD2 has better provided samples for metal wihtout any addon, but i recommend also there the "evil drums" pack. But keep in mind..every samples are raw...not preprocessed like slates.
So if you mainly use your own wav files, then you can't use Superior Drummer.
greets