A lot of people visit both in July and August. Personally it is too darn hot both places for me.
My family loves Hilton Head but after one June visit decided to skip summers for now on. Most RCI exchanges will not be on the beach which is ok but think 95 degrees, moderate to high humidity and no breeze. We visited the pool in the mornings. Stayed in the AC from 11- 4pm and then enjoyed the beach or other activities until sunset. Evenings weren't too bad. Unless you were in a boat on the water or in the AC the mid day was too hot.
Myrtle Beach has more RCI properties on the beach and has easier overall access to the beach. The activities in MB seem more entertainment (rides, go carts, waterparks, mini golf, shows, etc) centered than nature centered and most are in the AC or places with more of a breeze.
The nice thing about both places is the ocean is warm enough that you can stay in the ocean until sunset. In the MD, NY, and NJ beaches where I grew up, I can only stand being in the water during the hottest part of the day and by 3 its too cold.
Realistically you should be able to get a nice 2-3 bedroom at either place since you are planning early and have enough tpu's. Decide on your first picks and put in a search and keep an eye on the sightings board.
