To me, the biggest benefit would come from getting breakfasts at SPG properties in Europe and Asia. The SPG offerings in many of the cities I visit are superior to both the Hiltons and Marriotts; as a lowly Marriott Gold, though, I usually go with a FS Hilton or Marriott to get the breakie and lounge. The one big advantage along those lines of HH Diamond is when I travel with family- the Marriott tend to be stingy about lounge access for the kids, while the Hiltons
almost always give the whole family lounge access as well as breakfast in the restaurant (That was true back when I was HH Gold as well). That said, if you're only staying at properties in the US, you're losing out. The lounges and breakfasts elsewhere are quite a bit nicer (I can no longer bring myself to eat st the HGIbsmf Embassy Suites...)