We have stayed at both - although it's been a few years now.
VDP Cancun is a nicer facility (as Bill said, more opulent), has more and better food choices on site. The new sister resort the Garza Blanca next door is now complete and I would think, but don't know that the dining options there are also avialable. It wasn't finished when we were there. Beach is quieter, with very nice beach services. But there is seaweed, that is cleared constantly, once you walk into the water the bottom is more rocky than sandy (bring water shoes). During our stay we went to Isla Mujeres two days to use the beach there - really beautiful (on the west/north side?) rather than stay at the resort - easy ferry ride - either the slow car ferry right next door or an easy cab ride to fast ferry just a ways down the beach. VDP Cancun is remote. We did 3 days all-inclusive and 4 days not. If your preference is dining offsite know that it's a meaningful cab ride into town and a really long cab ride into the hotel strip. If you like all-inclusive and staying at the resort for your dining this is a really great option.
The Westin is perfectly nice and very upscale. Just that in comparison the resort is nicer at VDP Cancun. Very nice facilities, very nice/wide beach. I know I have read some information on the sargrass (sp?) issues in the recent past - not sure what the current status is. The ocean can be rougher over here...but totally depends on the weather. It was nice most of the time when we stayed here. No problem swimming most days. For us the biggest difference is the off-site dining options. We almost never do all-inclusive and prefer to explore off-site options. That's really easy here. Resort is in the midst of the hotel strip. Choices up and down the road. Several close enough to walk to, a shopping mall right across the street, many others a short cab ride. Also, while not at all our interest, the various niteclubs are all mostly located in the hotel strip.
Other than visiting Isla Mujeres we didn't do any off campus exploring or tourist activities so I can't comment on the access to those activities.
If a relaxing stay at the resort doing all-inclusive is your thing then I would go with VDP Cancun.
If going out for meals is more your thing then I think the Westin.
FWIW
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