When I had the iPhone 6 briefly, it was quite easy to use portrait but I never could(And still can't) understand how males with large hands were able to handle the small screen size of iPhone 5/5s and even the iPhone 4s and below lol.
Maybe because I am spoiled, my phone is 6 inches(So larger than an iPhone 6 plus) and it sort of makes everything else I use feel tiny lol but I got irate trying to type on an iPhone 5s because my thumbs had no room to move lol
Mostly portait. I go into landscape on certain responsive sites if I want to see more elements that are hidden for mobile (such as XenForo last post date on the forum index) or worse, sites that aren't responsive and I have to keep scrolling left/right in order to read sentences. Overall I'd say it's 99% portrait though.