Most Marriotts in the locations you mention are considered resorts and therefore do not have lounges, unlike Hilton. Additionally, they will not offer you a free breakfast in lieu of lounge access.

In my experience (and I'm Plat with Marriott and currently hold no status with Hilton), Hiltons treat you far better than Marriott in Hawaii. With no status we got many more perks from Hilton at the HHV than we did from Marriott at the JW Ihilani (same trip) and many more from Hilton at the HWV than from Marriott at the Marriott Waikoloa (same trip).

HWV offered us an upgrade to oceanview room and Executive Lounge access under their NOR1 program for $35/nt. I'm not sure if they still offer it but it was definitely worth it (although since you are Diamond you would seem not to need to pay for it). We were also able to combine the NOR1 with award nights.

In Mexico, neither the Marriott CasaMagna nor JW Marriott offers any kind of lounge for status, nor were we offered breakfast at the CasaMagna (though they did give us a very nice upgrade). I have an upcoming stay at the JW Cancun and expect to get little even as a Platinum.

If I were you, I'd stick with Hilton if you are looking for Exec Lounge perks in the islands.