I love the GH Kauai and have been there numerous times over the past 20 years. I've also been to HR Maui several times.

I've found Maui far better at offering upgrades on award stays. Kauai will likely offer you, as a Diamond member, a better "view" room, but I've been upgraded to suites on a couple of award stays in Maui, including one stay with 4 rooms booked.

I'm not willing to pay for a cabana, and the "tented" pool chairs tend to get grabbed very early each day around the Kauai pools. If you're on mainland time, it might not be a problem, but I wish the staff policed them better. I don't recall this being an issue at HR Maui.

I've generally loved the staff at the Kauai Grand Club, but agree with others it's declined in the last few years, and can get overrun with guests. Not my experience with Maui, but it could be the time of the year, hotels events, or ??

You didn't mention the beach as one of your criteria, but I agree with others that Maui wins here.