The 6+ is just too big to use with one hand. You can hold it fine with one hand, you can touch the side of the screen with one hand. So you can read without problem. But you can't interact with it with one hand.
On the other hand, the 6 is tricky. It feels like you can use it with one hand. But it's still very hard to reach the top of the screen - and you have a lot of functions you need all the time at the top of the screen on iOS. Also, you can reach the opposite side of the screen, but you have to strain your thumb. So, the 6 looks like you can use it with one hand, but it's not really comfortable. If you have both hands free, you will just use if with both hands, because it feels better...
This is getting me to reconsider. The main advantage of the 6 is one-handed use, the 6+ has anything else: big screen, more autonomy, split view... If you're going to use both two-handed anyway, might as well get the 6+...