From my personal (and limited) experience a pithy summary: PEI is more civilised, Cape Breton is more authentic. Obtaining gigs would be more likely on PEI, I would think (it's bigger, with more people). To experience traditional music and dance where it lives, go to Cape Breton, where it's less swamped by tourist trash. Very different places to see, too. Depends on what you want and the time you have.

~ Becky in Tucson