I spent entirely too much time playing with both phones in the Apple store today. Still have no idea which I want to buy.
I spent the most time with the Plus. It feels comically large for a phone. I feel the design needs to move towards feeling like an iPad Nano rather than a Supersized iPhone. Subjective, but I could and would read comics on it. However, I don't think it's worth picking up just for that - it's still an inferior comic-reading experience to my ancient gen 1 iPad (or an iPad Mini). But combined with the battery life, it becomes a much more attractive proposition. I could type one-handed fairly easily, I didn't find it at all slippery, and Reachability works perfectly once you realize the Touch ID button is a less sensitive to your finger than the screen.
However, I really didn't care much for the design. As I say, it feels like a giant phone. Where the 6 seemed gorgeous, the Plus actually felt tacky to me. Not a fan of the gold, and the space grey's black Touch ID button looks like cheap plastic. The silver probably looked the best, but after much testing I found the black bezel displayed content much less obtrusively - that's one I'd pick.
The three 6s were not only somehow beautiful phones despite identical aesthetics to the Plus; they also felt absolutely perfect in the hand. Even the antenna bands seemed less offensive. I wouldn't choose to read comics on the 4.7" screen, but I was more than satisfied with everything else. Videos, Safari, Photos...everything felt large and vibrant, and sized up enough to share without forcing others to crowd around your typical tiny iPhone screen. I can see apps like Garage Band benefitting from the 5.5" screen, but I'd rather use a desktop for that kind of task.
Part of me is leaning towards the 4.7" 6 just because it felt more like a premium product. If Apple had translated the sense of quality inherent in their iPad line to the Plus, I'd be all over it; size be damned. Or if they'd made the 6 a little thicker and given it comparable battery life to the Plus, I'd jump on the smaller phone without a thought. As is, I'm still humming and hawing as I reflect on two imperfect products. Leaning towards the 4.7" 6 this time, but my eye will be on the 5.5" as the design evolves over the next couple of product cycles.