I have been to both. The beach and view from room is better at the Princeville. The food is better at the Ritz Maui. It'll probably be easier to secure an upgrade at Princeville if you are SPG Platinum but I wasn't one so I don't really know. I would rate the standard rooms at both properties about equal. Also bear in mind that both properties are far (>45min) from the respective airports. But I felt the Princeville was perfectly situated whereas the Ritz seemed to be far removed from everything in Maui. But that's a personal observation.

It's a personal choice but IMO, Kaua'i is far more romantic than Maui. Maui is almost like Honolulu - built up and touristy. Kaua'i is lush green, fairly undeveloped and has amazing scenery (helicopter tour is a must!) esp. the Princeville area, the Na Pali coast and Waimea Canyon. There's far more to do in Maui though so if you want do experience the islands, Maui might be better.

That said, I would go back to Princeville in a heartbeat