Outside the USA, Sheraton is considered a very good brand that is just below luxury; outside the USA, Marriott is much less known and isn’t known to be as good. In the USA, Marriott is considered more reliably solid than Sheraton, though in recent years there has been a massive renovation program for many Sheratons.

Why the Frankfurt hotel went with both brands is curious and not likely to be known any time soon, but I would expect a bit more personality with the Sheraton and a bit more of the solid but reliable quality with the Marriott. Perhaps Marriott corporate wants to showcase Marriott being related to the better perceived Sheraton Internation brand? Perhaps the owner wanted two premium brands for their own marketing or corporate/group business ends? Who cares really?