I've recently stayed at both the S-L and Conrad, but not the Marriott, so I can't compare the rooms/service between Conrad and Marriott. The Marriott's public areas, however, appeared to me to be below the Conrad's standard.

I would agree with you about the Shangri-La's restaurants being clearly better than the Conrad's, but I found the service and room quality at the Conrad and Shangri-La to be very comparable (and exceptionally good) -- if it matters, I'm in both Golden Circle and HHonors. I was especially impressed with the service in Conrad's Exec lounge, which Ms. Connie Cheung oversees with consummate skill and care.