Its easier to get to Hilton Diamond than Marriott Gold/Platinum if most of your RONs are one night...otherwise you've got to hop around hotels to accumulate stays under HHonors.
My experience has been the service at both types of properties is good, although many Hilton properties in the last year have started offering you a choice of Diamond benefits when you should be receiving them all. Also had issues getting bottled water at a few Hilton properties, to the point where I had to walk down and get them myself (not really that big a deal).
Bottom line? I think they're about the same.