I used to live on Oahu, and went to Maui a few times. We loved Wailea for its peace and quiet. Kaanapali is fun, but has lots of traffic (think smaller version of Waikiki), but Lahaina is quaint. Kapalua can be very cool and windy - we spent a Thanksgiving week at the Ritz there, but wore long pants and sweaters the whole time. We loved it - reminded us of the mainland - but some guests actually checked out and went down to Wailea (the staff said that's not uncommon). So if sun and warmth is important, I'd avoid Kapalua.
For a honeymoon, I'd go with Kaanapali or Wailea.
If the Westin is near the older Sheraton, that is a very beautiful area of Kaanapali!