Hi Gosomewhere, welcome to FlyerTalk!
I've been to both properties. I concur with the sentiment that the Sheraton Maui is dated/tired. As a Platinum member, I received an upgrade to an old terrible, smelly, obviously moldy suite. After the first night I got moved to a better suite in a better location with a nice view of the ocean, but it was still quite dated in many ways. It was fine, but not excellent by any stretch. The best thing about this property is that it has the largest beach of all the Kaanapali resorts. Many of them, such as the Westin or Hyatt have little-to-no beach in front of them, as they have been increasingly washing away. So, if beach is important to you and you don't mind an average property, the Sheraton would probably be the choice.
Personally, I like the Westin Whistler a lot. If skiing, you cannot beat the location. The rooms are very nice, many suites and I've been upgraded both times I've been there to two-bedroom suites with a living room, kitchen/dining table area. Very nice.
Given the varying reports on Platinum upgrades at both of these properties, I would not expect much as a Gold member. If you have your heart set on a nice room, you'd best book it straight-away and not rely on an upgrade. You're likely to be uphappy with the result.
Comparing the two hotels, I like virtually everything about the Westin Whistler more than the Sheraton Maui. It's too bad you're doing them in the order you are. I think you will feel like you are stepping down.
There has been a lot written on FlyerTalk about both of these properties. Up to the right is a "search" button. Click on that and search for "Sheraton Maui" and then again for "Westin Whistler" and you will find lots and lots of good information about both properties.
Cheers. I hope you enjoy FT!
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AZ Travels the World
Moderator for Starwood and America West