As a die hard MR Platinum with the Marriott and Westin showing similar prices, I would definitely go with the Westin even if it was more expensive. Much nicer in my opinion. I have stayed at the Marriott twice (my 1st trip to Cayman and 1 night before departing on the Cayman Aggressor Live Aboard) and the Westin at least a half dozen times. The Westin's pool is much larger and nicer, swim up bar, not as squeezed in as the Marriott, nicer area of the beach, full Spa in separate building whereas Marriott allocated a couple of their multi-purpose rooms for use as a make shift Spa.
$160 advance purchase rate for Marriott, $159 summer spectacular rate for Westin (3 night deposit $477 no refund 24 hours after booking).
AAA rate - $171 Marriott/$201 Westin
Westin has a $20 resort fee but I couldn't find what that includes anywhere on their site.
Marriott: Renovation Information
* Our property will undergo balcony enhancements from December 20, 2008 through November 30, 2009. Some noise and disturbance may be experienced by guests in certain areas.
Westin: Casa Havana (their high end restaurant) is closed Aug 15 - Oct 15.
Another option is to use points and cash in a PointSaver award at the Cayman Ritz Carlton which is no doubt a better hotel than either of those properties. Here is the info on Marriott. com:
https://www.marriott.com/rewards/rit...tsaversrewards
You will need to call Marriott Reservations to book A RC PointSaver award.
Note: I will be staying at the Grand Cayman RC for the first time this coming November.