The question "on what orientation do people prefer to hold their tablets at?" is very similar to the question: "Do people prefer landscape oriented photographs or portrait oriented photographs"? There is no correct answer to this question since it depends entirely on the actual picture. In case of tablets: it depend on the application. In contrast to a TV or a movie theater, a tablet is by definition a device with which you can easily adapt the orientation depending on the application. Some applications will do better with portrait mode, others will do better in landscape mode, others can make good use of both orientations if well implemented. So the best answer to your question is: users will prefer to hold the tablet in the orientation the application was designed for.
Now, over to your real question: "is it good UX to have the portrait mode show a detailed view of a selected item while the landscape mode shows an overview?"
In my opinion, viewing the details of a selected item is best done by... selecting the item. Not by rotating the screen.