Elliote Rusty Howard puts it laconic: "RELAX Wins" and "the debate is effectively over".
Unfortunately posts like his affect many people, keeping them from deciding on their own behalf.
Actually, I used to prefer Relax NG compact syntax over XML Schema Definition from the textual point of view. But I learned recently, that the XSD tool support is excellent for XSDs compared to the poorer support for RNG/RNC.
That's why - although I started a project using RNC - I switched to XSD, just in order to take advantages out of the tooling. I'm not a fan of Eclipse, but its XSD editor allowes a lot better modelling than any RNG/RNC toolset I found.
By the way, Mr. Howard: How many projects do you really do in let's say 3 years? And what do you think, how many projects are there currently, you don't know?