While I don't know any codes other than M11 I have stayed at both properties. The Casa Magna is great and the new JW is massive an and a bit overkill IMO as well as the Ritz. My personal favorite preference is the Fiesta Americana Coral Beach. The rooms are fantastic! and all ocean front. You do run the risk of not getting an ocean front at the Marriott...
Good Luck and post another question, as I have at quite a few properties in Cancun (with the Coral Beach still being my favorite).
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