Both are great and both are beach front. It depends on whether you want to be in the middle of the action or not.
Marriott is in the middle of resort row, so it's surrounded by other resorts. It's on an oceanfront path that runs the length of Ka'anapali Beach. It tends to be more crowded and energetic. You can walk to nearby resorts and restaurants if you want. If I was renting here, I'd look at the towers (Napili or Lahaina?) because they were built to be timeshares and have proper kitchens and en-suite washers/dryers. The older section was converted from hotel rooms, so they have kitchenettes and shared laundry facilities.
Westin is just off resort row, so it's quieter and the beach isn't as crowded. It's on an oceanfront path that meanders on an off the ocean, so it was more of an afterthought than purposefully built. The villas are very nice and built to be timeshares, though the north phase lacks a stove. Snorkeling is outstanding in front of the resort. They offer a very convenient shuttle that goes between Lahaina/Whaler's Village and the Westin, so no worries about drinking and driving.
Both have great pools. Both are beautifully landscaped.
Personally, I'd look to get the best deal and go from there.