My experience is similar to Merry Flyer's. I am also Diamond VIP with Hilton and Black Marquis with the Marriott. Hilton International/Conrad properties are superior to Marriott properties. I also get upgraded to a better room about 90% of the time in Hilton International/Conrad properties.

In the US, most Marriott properties are superior to Hilton properties. I get room upgrades more frequently with the Marriott (even though I am only a black level member, which is their second highest tier). In fact, in some Hilton properties, they don't even know what a Diamond VIP is. Some of them think Diamond is lower than Silver because they don't see a lot of them. Only after explaining what Diamond VIP is do I usually get a room upgrade. Anyone else with the same experience?