I look at it like this:
What do you use it for? Do you PRIMARILY use your phone as a phone, or as a multimedia / text based communication device? And if you primarily use your phone as a phone, do you use a hands-free or bluetooth headset?
Pocketability is a big factor, but unless you have your jeans painted on you before you leave the house you should most likely be able to fit it in a pocket without too much issue unless you are very petite. So that can take that question off the table.
I don't like holding a brick up to my face to have a conversation so that would be my questioning factor - the 6+ is pretty big feeling (if you're unaccustomed) when held up to your ear to speak on the phone. If you use a headset, that takes that question off of the table as well.
Then it comes down to one hand use to two hand use. I myself use my phone 2 handed 95% of the time regardless of size. That started back in my blackberry days, too. It's more comfortable for me to type with both thumbs. I use it one handed to:
1. unlock my phone (both the swipe and 4 digit code is one handed)
2. check an app that requires no interaction - something that only takes a couple taps like checking the weather or opening an email.
I'm also fairly proficient with either hand so if something is slightly out of reach with my left hand, I can toss the phone to my right hand to reach it. So to me, that kind of takes that question off of the table.
I'm really, really interested in the 6+ battery more than any other factor out there. There is something to be said about the significantly better battery, and as much as I'm on my phone all the time, that's what's really tipping me towards the 6+