Proud owner of a 6+ and iPad Mini 4 here! I find that the iPhone is both too big and too little for what I like out of it, and am honestly thinking of upgrading to a regular-sized 7 when they come out. As nice as the larger screen is, I absolutely despise wide screens of any kind and find that the stock landscape keyboard on the 6+ is one of the most horrible typing experiences I've ever used. I also don't use it for much but videos or the occasional picture anyway.
Not so with the Mini. Even with the iBlason case on it, I can thumb-type at nearly desktop keyboard speeds while holding it in both hands. It's got the right balance of screen size to the point that it just feels comfortable to hold and type on while still being nice and roomy enough to watch videos or read from. The 4:3 aspect ratio goes together with reading like peanut butter goes with jelly... Try reading a book on one of those cheapo 16:9 Android tablets in Portrait mode and see what I mean.
Apps just look better and feel more productive in an iPad format. You can see all the controls you need to without them being buried in two or three sub menus and all the information is at your fingertips. I find myself being a lot more productive and tracking my diet, scheduling appointments and meetings, and taking notes and lists as needed... Much much more than I do on my 6+.
Yes, I understand some people will be perfectly fine with the 6+ for their needs, or even just the basic 6... But for me, the experience of a just-right screen is what I've been looking for in a tablet all along.