My big fancy prediction about customer satisfaction:
During the first week or two, the people who chose the 6 are going to be the happiest. While they are adjusting to the increased size in their iPhones, they are going to think, "Well, at least I didn't pick the even bigger one." And they are going to love having a bigger screen than before.
Meanwhile, the 6+ guys (I assume they'll mostly be guys), are really going to love their screens. But while they're adjusting to the feel of the phone, they're going to be wondering if they should have chosen the smaller 6. Some won't be able to get past the second-guessing and will trade it back in for the smaller.
A month in, those that stuck with the 6+ are going to be really happy with their choice. The others will still be happy, but if anyone is second-guessing their decision, it'll be some of the 6 owners, wondering if they should have gone bigger with the better camera and battery.
But they're all going to be generally happy, especially when they pick up an older iphone and wonder how they ever used it.