We just came back from Hawaii with 5 nights at the Westin Hapuna Beach Resort, 2 nights at Princeville Resort and 6 nights at GH Kauai, I've also previously stayed at Mauna Kea and Westin Princevile Resort 3 years ago.
We liked the Westin Hapuna Beach Resort, more relaxing and less crowded than Mauna Kea. All rooms have at least partial ocean view at the Westin, not so at Mauna Kea (my previous room had garden view as MR Gold at the time). As Plat, we had free full breakfast buffet for 2 each day which was very good. Breakfast buffet is $37. We walked over to Mauna Kea and found the beach to be more crowded, smaller and not as good for snorkeling than Hapuna Beach.
The weather was very good on the Big Island week before Christmas, occasional afternoon clouds and showers one afternoon (out of 5). There is enough to do and plenty of nice spots for snorkeling on the Big Island. Kauai forecast was rain everyday for 2 weeks, although we only ended up with 2 full days of rain out of 8. Princeville weather was terrible, one day of sun and rain pretty much everyday, not just afternoon showers. Surf conditions in Kauai around New Years were rough with steady winds, beaches were all flying red flags. Snorkeling at Poipu was decent with lots of fish, but crowded. There were 3 Monk Seals and 4 turtles that took turns resting/sleeping on Poipu Beach. Road west of Hanalei Bay is still closed (restricted to locals only) since the April flood. This meant the popular beaches on the North Shore are off limits.
If you are going over the winter, I would recommend The Westin Hapuna Beach Resort. The free breakfast for Plat is much better than breakfast at the GH Kauai Grand Club for Globalist. Although no resort fee and free parking is great at GH Kauai for Globalist.