Both are wonderful destinations.

I've been to Nova Scotia twice, first time rented a convertible and did the cabot trail - just wonderful and did see a moose followed by 1.5 days in Halifax/Peggy's Cove. Second time was just a weekend in Halifax with side trips to Peggy's Cove and Lunenburg.

Last summer spent 10 days in Newfoundland (mid July) - could have easily doubled that. Did St. John's, Bay of Fundy area and Fogo island. Whale watching and iceberg watching were the highlights. You will likely not see any icebergs in August. NF requires more careful planning in terms of accommodations and distances are huge. We didn't even remotely scratch Gros Morne NP, so I would definitely want to go back.

Regarding car rentals - yes, in NF there was a hefty one-way rental fee of 300$ or so. Not sure if the same applies to NS - we flew in and out of Halifax both times so this wouldn't have been applicable.