While some will extole the virtues of one or the other (90 or 96), I would say pick one COE and stay with it. I really think most people who fuse will be happy with either. The problems created by using both (separating them) are more daunting than just choosing one.
I use exclusively 90. I use Bullseye (mostly) and Uro (90) and Wasser. Never had any compatibility problems. One of the things people tout in Spectrum is that it is less expensive. In small (pendant sized) pieces, that really is not a factor.
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