I don't think you will have any problems getting sites after Labour Day, it gets quieter in September.
In terms of choices, Rabbit Blanket is "inland" on a small lake,it has (from memory) about 30 electrical sites on the first loop, you can rent a canoe from a rack on the lake shore near the office and you can paddle into another nearby lake and portage as well, the lake is small and calm. The small lakes nearby also provide some canoe opportunities and there is a rental rack on one of them. RB is more sheltered from the Superior winds.
Agawa is on Superior so it will give you that experience (don't think they rent canoes there), it has more sites and a visitor center.
Why don't you try both, stay a couple of days at each one? You can also camp in one and go and check out the other. Just be sure to be on the top half of your gas tank, no gas in the PP, only gas is at Pankcake bay or Wawa, well over 100 km. apart