There are Pearls and Pearls!....There are Marbles and Marbles!
For me there is nothing in it, I like them both exactly the same.
The trouble is there are more average Marbles and Pearls around than high quality specimens, top quality examples of both are right up there with your best black rays......That's my honest opinion.
I love quality black rays but I feel the ban from Brazil sent everyone crazy once they re-appeared, now I would say it is far easier to find a high quality black ray than it is to find high end Marbles and Pearls.