I'm going with the iPhone 6 Plus because:
1. I wanted a bigger screen (>5 inches)
2. It has a better display (401 ppi)
3. It (supposedly) has better better life
4. It has a better camera (with OIS)
The decision was a little easier for me when I realized that I don't ever talk on the phone, texting and pictures is most important.
These comparisons might help..
http://techcrunch.com/2014/09/09/hands-on-with-the-iphone-6-and-the-iphone-6-plus/
This actually helped me with my decision. 5.5 it is.