I'm headed to Newfoundland next week from NYC. Never been to either, but I'd look into costs for car rental and lodging availability in NL in August if I were you. From what I've read, there may actually be no more cars left to rent on the island if you haven't reserved one yet. Nova Scotia may be easier to plan last minute (which I realize it's not, yet, but things seem to book up early in newfoundland). But both should be great trips, whichever you choose!