I have yet to buy either but I'm dead set on getting the 6 Plus soon just based on my Apple Store experience. I played with the 6 Plus for 45 min and the 6 for 5. The 6 Plus has so many more advantages over the 6 that I didn't even care about the 6 within minutes of using the 6 Plus.
Landscape mode system wide
- Safari Tabs like iPad, bookmarks shown while browsing sites (dual pane)
- Extra buttons on landscape keyboard (arrows even)
- Dual pane apps (Messages, Mail, Settings) like iPad
Messages
- Contact List Pictures in list view. No more accidentally sending messages to the wrong person (boss!) for those who are visual. Believe it or not, this is fairly common and I've had friends do it and I've done it once by accident.
Camera
- Optical Image Stabilization for much clearer low light pictures.
Screen resolution
- The resolution is higher (it's sharper) and the screen resolution is a lot larger (52% more screen real estate than the 6 and 65% more than the 5S, math might be off, still screen resolution is quite a bit larger).
- Video looks fantastic in 1080p on that screen.
Battery Life
- For many this is the sole reason to get a Plus over a 6. The Battery life will easily last more than a day of normal usage.
Size
- I did the Levi jeans test and it fit in my 505's pockets (they're not skinny jeans but not relaxed either. They have a tailored look). As long as your pockets aren't short like women's it'll fit fine.
To be honest, the only thing I didn't really like was that the landscape keyboard was basically impossible to type on as the keyboard itself is in the middle of the screen. Why they didn't add a split keyboard option like on the iPad is beyond me (and a feature I've already requested).
I didn't find the 6 to be much of an upgrade from the 5 I have. Sure it has stabilized video (which is freaking amazing if you haven't seen it) and a slightly larger screen but all of the great software features is 6 Plus only.