Landscape all the way. I love the landscape mode so much on the plus I find myself dying to do the same on the 6, then realise it's not there #. I use my phone to death in work and out, and landscaping with 2 hands is much more comfortable for long usage sessions. Although I'll say this; using the keyboard in portrait on the plus is an epic experience. No more hitting the wrong keys on the edge of the screen because of big thumbs!
I would love to use landscape more, but many websites offer a mobile version that doesn't work well in landscape mode. There are also many apps that only work in portrait mode, so I guess most of the time I use my iPhone in portrait mode and when watching videos and pictures in landscape. Also in Safari, for websites that work better in landscape, I use it.