Forget the fact that Xi, like classic OPC, is vendor specific (depends on Microsoft technology). The bigger point is how many standards do we need? It's already to the point of madness. Isn't that the point of OPC standardization?
OPC-UA is now an accomplished fact. We recently attended the OPC-Interop conference in Nuremberg, Germany and were pleased with the heavy attendance. In fact, we successfully tested Ignition OPC-UA against nine other UA servers and ten other UA clients. Commercial products are now available from a large and ever growing number of vendors.
The more telling point is that OPC-Xi has no validation testing available (to my knowledge). And there are no interop conferences to ensure compatibility (also to my knowledge - correct me if I'm wrong). So is there really interoperability? I don't know. Take your chances.
NEWS FLASH – OPC-Xi is DEAD! Actually, it has been renamed to OPC .NET 3.0 (WCF Edition). Man, that is a mouthful to pronounce. As I was writing this one of our developers pointed out this recent change to me. By the way, WCF is the new Microsoft replacement for DCOM.