No doubt this will muddy the waters, but...
My preference for both iPad & iPhone is to use portrait mode, for several reasons:
1. If I am typing, the on-screen keyboard occupies about a third of the screen in portrait mode, but about half the screen in landscape mode. It therefore sometimes prevents me seeing what I have typed!
2. It's easier to hold in one hand, with the centre of mass closer to the wrist.
3. Small text isn't really an issue for me, as I am shortsighted, and tend to take off my specs and hold either device about 30cm from my face. Since both devices try to optimise to the viewing width, there's more viewable 'real estate' visible when in portrait.
4. I find it easier to read a greater number of shorter lines, rather than fewer but longer lines. (Maybe that's why most books are printed in portrait mode, and newspapers are printed in columns?) The Kitz site is a prime example of this.
Of course, the opposite applies when watching full-screen 'YouTube' videos, for example, because they are optimised for landscape viewing, and require little user interaction.
Since the screen dimension ratios are so different between the two devices, it doesn't surprise me that some apps are restricted to either landscape or portrait mode. The work involved in trying to optimise for both orientations on both devices must be significant!
TBH I rarely use the phone for anything other than calls, texts and messages, so my useage is probably atypical, and my opinions should not be given too much weight!
