I was in an Apple store for about an hour today, and I still cannot decide between the 6 and 6 Plus.
6 Plus can't be used with one hand, period. Reaching all the way to the far left of the screen feels very difficult and unnatural, and using the keyboard with one thumb is close to impossible unless you have gargantuan hands. But that said, I found using the regular 6 with one hand to be a bit fiddly too (but possible). I guess I'm just accustomed to by iPhone 4S. So it's best to use two hands for either one, and I found the 6 Plus to be the more comfortable of the two when using two hands. And the keyboard on the 6 Plus is amazing if using two hands. Because of the sizes of the keys, I could type quickly and accurately.
I own a 4S and an iPad 2, and I'm probably going to sell both to get a 6 or 6 Plus. The 6 Plus would be a nice happy medium, but I have a hard time coming to terms with how big it is as a phone. It would certainly make reading and playing games better than the 6, but it strikes me as a bit cumbersome for a device that we're used to being small and convenient. Holding it up as if to make a call felt weird.
Grah! The more I think about it, the harder I find it to decide! Help me out, GAF? How did you guys decide?
Oh, and is there any possibility of app devs going back and making some iPad apps compatible with the 6 Plus? Or future apps being compatible with 6 Plus, but not 6? That could sway me.