I had a 6 initially. I originally wanted the 6+ but all I could find in the stores was a 6 so I "settled" for that. I ended up having to take it back to get replaced for some display issues and randomly asked if they had any 6+ models in stock and they did so I returned my 6 and got the 6+. I'm much happier with it and here are a few observations:
1. Size - IMO, the 6 felt better in hand. The 6+ did take some getting used to and will take adjustment coming from the 5/5S. Once you get used to it, it's not so bad. I think it's worth adjusting for it to experience the advantages it offers over the 6.
2. Display - Aside from the white hot spots my 6 display had, I didn't have many complaints. However, I could discern some pixelation when using it at normal distance. Going to the 6+ it just trumps the 6 in screen real estate and resolution. I have yet to see any pixels on this thing. It's awesome. When it comes to display "quality" I found them both comparable. However, it does seem Apple is having a lot more QC issues with the 6 panels where as for the most part the 6+ displays have been more consistent. This could just be due to the fact that they produced a lot more of the 6 vs the 6+ at this point.
3. Battery - I didn't really put the 6 through its paces but the battery life seemed about 20-30% better than my 2 year-old iPhone 5. The 6+ just keeps going and going and never seems to die. It's taken some adjustment to get used to not having to charge my phone at the end of every day. The stand-by usage is phenomenonal on the 6+. If you are doing a lot of "screen-on" tasks at high brightness however, you won't see a major improvement. Clearly, the display is the biggest battery hog.
4. Camera - Both take great pictures and not being that into photography, I can't really tell a difference. I did notice that the OIS works in the 6+ but haven't taken enough footage with it to make any substantial claims that it makes the camera significantly better than the 6.
Those are the main points of difference between the two that I wanted to highlight. I feel as though if I kept my 6 I would not have been happy and would be constantly feeling like I settled which definitely would have continued to bother me during my ownership. Since I did get to experience both, I don't have to feel any "what if's"