They're both on Kaanapali Beach but the Marriott is on the south end of Black Rock. That area is resort row, with lots of high rise hotels and shopping and lots more "action." There's also an ocean front path for walking/jogging between resorts, which is a real plus. Grounds and pool are awesome. Rooms are hit and miss based on when they were updated. If you have teenagers, they'd probably enjoy it here better than the Westin.
Cons: With the exception of the new towers, the villas are smaller because they're converted hotel rooms. They have no washer/dryer in the unit (but nearby) nor do they have full kitchens. (The new towers, on the other hand, do have all the amenities you'd expect in a TS.)
I own at the Westin. It's on the north end of Black Rock, which is a more laid back area of West Maui. The villas are beautiful. The grounds are beautiful. The pools are great. The beach is great and there's great snorkeling right there. They provide a complimentary shuttle which runs between the Westin villas, the Sheraton, the Westin hotel, Lahaina, and Whalers Village. That made going to Lahaina for dinner/shopping very convenient as you didn't have to find a place to park. All rooms have a w/d and full kitchen (though the north property lacks a traditional oven). Property is new so the rooms still look great. Bathrooms are huge. Heavenly beds are awesome.
Cons: No ocean front path so you can't walk as easily to neighboring restaurants/shopping. Not as much action here. Studios at the south property don't have lanais. Only TWO tennis courts.
Good luck with your decision. It's not easy, but I have a really hard time believing that you'll regret your choice no matter where you pick.