When I was upgrading in January, I was torn between the two. I ended up getting the iPhone 6 and loved it. Two weeks ago, I switched wireless providers and had the opportunity to choose again. I decided to live a little and go with the Plus this time around. So far I really like it. It is much larger than the 6. It takes about a week to learn how to hold it one handed and if you have small or medium hands (I have medium), it can be difficult to reach the corner with your thumb. For example, if I hold it in my right hand I can text with my thumb but when I have to reach the ABC key in the far left bottom corner to insert numbers, I have to really stretch and in turn make sure I don't drop the phone in the process.
The battery life is easily twice as good on the 6 plus vs the 6. The 6 isn't bad, but the plus is damn near impossible to kill in a day. I was traveling for work on Thursday and used mine from 4am to 11pm and still had 22 percent battery remaining. I ran the gps for over 4 hours (with display on), I watched a movie, I watched 30 minutes of you tube videos, responded to about 30 emails, sent and received over 100 texts, and got constant group me notifications and still didn't have to charge.
What I noticed after using it for a week or so, is I have subconsciously started using two hands to operate the 6 plus. The only time that is annoying is if I am sitting at a red light(I know, no texting in the car), or have only one hand free and need to respond to a text.
In the end, I am really happy with the 6 Plus. I find myself using it almost exclusively when surfing the web at home or on the porch with my morning coffee whereas before, I would use my iPad. I still choose to watch movies and such on my iPad since a 9.7 inch display always beats a 5.5. That being said, it is much more comfortable to watch videos on the 6 plus vs the 6. The 6 still feels cramped, or feels like a cell phone whereas the 6 plus really does feel like a mobile computer.
If you absolutely need comfortable one hand operation on your device (and don't have NBA sized hands) get the 6. If you want better battery life, a larger and much sharper display, and an overall more immersive experience, get the 6 plus. To me, you are spending the same money on a data plan monthly so you mine as well get a device that offers a better entertainment experience.