Odd question really as I never think about it, just turn to get more on screen.
Graphically horizontal is more instinctive, as eyes are not vertical, but for the written word it's always portrait as it's the common page layout. Nature and nurture in harmony.....
E-books, yes, definitely portrait. Web browsing-if searching, I may start with portrait, but if doing posting on a forum-switch to landscape. With apps, it's 50/50 depending on the app.So my both vote applies for sure.
Landscape is my preference for most things except for reading. There are some apps that are portrait only so have to use it for them, and some note taking just "feels" more natural in portrait.
It's a good thing I check back through 8 pages of largely "moved" threads. I was about to post the same poll.
Suggestion to Admins. Put a time limit on the moved thread notification in most cases a couple days is all that's required for the OP to find where the thread went. Nobody else will care because they will generally find it in the destination location anyway.