We've strayed far from the OP's question of where a Gold is better off earning. I'm still convinced that at the Gold level, if we're talking normal business trips where the brands are pretty comparable, you're better off earning at Marriott. Most people, most situations, independent of unusual considerations (e.g., hotel-hopping in December to reach SPG Plat or something like that). I'd say this is true even if your entire goal is to eventually redeem at the best St. Regis or LC in the system.

To the Plat100's out there: I totally hope the unique programs for the top-tier road warriors somehow remain in the combined program. Marriott has made comments to date that at least show they recognize how passionate Starwood's top-tier is about their program. They've hinted that took them by a bit by surprise (perhaps because a lot of Marriott elites *don't* feel a terribly strong connection due to on-property recognition). Since they're clearly taking a loooooooong time to do this integration, maybe they'll get this part right. My hope would be that a 100-night level would exist with an Ambassador-type program, and that this would not be attainable simply through a premium credit card.