I always end up buying both and cannot decide. On one hand I like the better features (screen size, resolution, camera, and battery) of the +'s, but the size of the iPhone is more versatile for gym, one-handed use. I wish there was a way to have the two devices act as one. Meaning whatever was done/updated on one device was always backed up in iCloud and showed on the other device in real-time. If that worked and I could seamlessly switch between them on my Apple Watch it would be the perfect solution.
I switched back to my 6s today, but so far have only ordered the 7+.