Unfortunately, the question you ask is a very subjective one. A lot will depend on your tolerance for crowds as Hawai’i is a popular destination. That being said, I always travel during peak times (holiday/summer) and have never visited a site that was so packed that I didn’t enjoy myself (WDW/Aulani). The only time you may notice how crowded Aulani is is at the pool. As OP have said in other threads, you may have to get up early to get your pool chairs, but there are normally chairs at the beach. I’ve visited Aulani over July 4th and Christmas and neither time did I feel that the resort fullness impacted my enjoyment of my vacation. You just have to pack your patience and decide what you want your itinerary to be.
While I haven’t stayed at the Grand Wailea, I have been there for dinner while on Maui. This is a large resort and as such you probably won’t notice how many people are there until you go to the pool. They also have a beach, but of course all Hawai’i beaches are public. At no time did I feel the public spaces of the resort were crowded and I was actually surprised at how few people were in the restaurant for dinner.
If you haven’t already, check out the TripAdvisor reviews for both resorts during the month you want to visit to see if overcrowding is a theme. Good luck with your decision.