I can't speak much about Maui, but I can tell you that Kauai, even Poipu, is cooler than Oahu during the winter.
Now cooler is relative... it might be in the mid-80s in Waikiki and the 70s in Poipu. It's also more likely to be raining in Kauai in the winter than on Oahu, though when it starts raining in Oahu... it RAINS. Like for a week
I've been to Oahu numerous times in the winter, and Kauai 3 times in the winter, and when switching between those islands I found the temperature difference noticable.
Again, not bad. It was just noticable.
If you're looking for warm, dry & sunny in the winter months, you're best bet might be staying at the Ihilani on Oahu's west cost.