I've made that drive many times. It's eight hours from Maine to Halifax and it's not too exciting a drive, but can be beautiful if you're not the one driving.
PEI is pretty boring, not worth it...but Cape Breton is stunningly beautiful at the right times of the year. In February, I don't think it's going to be very pretty though. In September, however, I'd prioritize it over Maine.
I hope you have snow tires and experience driving in snow. If not, you need to consider that if going in February-March.
During the summer, Halifax is a lot of fun. The water front, the Citadel, Chickenburger, Freddies Fish House, the downtown farmer's market, Alexander Keith's....all worth visiting. Taking in a Mooseheads game if they are still playing and hoping to win the 50-50. Heck just going to a Canadian Tire store can be fun.