I've not used either for pens, but I have finished a few vulcanite pipe stems by wet sanding through 2000 grit, and then using plastic polishing compounds and a buffing setup.
If you have a sporting goods store in your area that sells bowling supplies, Ebonite has a special polish just for that purpose. Black PR can be micro sanded to 12,000 and than a plastic ploish such One-Srep added to buff to a shine. Jim S
I've only done a little ebonite, but I polish it the same as acrylics. No CA required for either.
I will say that occasionally, there are pits in ebonite. In that case, it's ok ti fill the pits with CA, then sand smooth, and polish like regular acrylics.