I have made a strange observation after extensive reading on an iPhone and then an iPad. I am most comfortable reading on an iPhone in portrait orientation, with night theme, and turning pages instead of scrolling. With the iPad, on the other hand, I like to read books with lots of text in landscape mode, with the day theme (i.e. white background), and using scrolling instead of page turning.
I have yet to figure out why that is so. I think the iPad resembles the PC form factor where reading is done on a wider screen. Then there is the kind of reading that is very iPad specific, such as reading a magazine. That is when I usually read in the portrait mode.