So I got some more information:

Spoke with the "mattress specialist" at the W Stores and was informed that
- The plush version is what the hotel uses
- The pillowtop version is, for all intents and purposes, the Westin Heavenly
- A box spring is not required with a proper platform bed, though he suggested that it would feel "firmer," in which case he recommended the featherbed to offset that

I also checked out some local mattress store offerings. The salesperson confirmed that pillowtop beds in general are notorious for flattening over time, so it's not something unique to the W/Westin models.