Please read this with the knowledge that this is a very subjective and very personal choice.
I originally got the 6+ over the 6 because I wanted the longer battery life and the OIS in the camera. It was my intention that I would use my phone more and my (at the time) iPad rMini less and maybe skip the iPad altogether eventually...
The problem I found with the 6+ was that I am a phone in my front pants/jeans pocket guy. It was always in the way, or uncomfortable when crouching down or bending over, putting on shoes, etc. In the end, I couldn't justify it, because the screen wasn't large enough to make it that much better for doing things on, reading, web browsing, etc. I was excited for the landscape mode on the 6+ for apps but after a while it actually just became irritating instead, with UI buttons compromised, and the dual column space making it seem actually more cramped that it actually was. I actually found myself using my phone less because using it in portrait mode like one has been used to with the smaller iPhone's was a pain, and using it in landscape mode was unsatisfying compared to the iPad mini.
The iPad mini size hit that sweet spot for me, and has done since the first mini was released. I found the norma iPad to be slightly too large for easy typing and true on the fly one-handed use. So when the new Macbook was announced and I decided to get it to replace my 2011 11" MBA, I used that as an excuse to go back to a similar setup that was my favorite personal gadget setup (11" MBA, iPad mini, iPhone 5) and gave the 6+ to my wife (she's a phone in her purse person, and has always coveted my 6+ anyway) sold her 6 (both gold) and got myself a new SG 6. I also sold my iPad Air 2 and ordered a new SG iPad mini 3, which I am now happily typing this post on.
All this is with the background that the iPad has replaced the laptop form factor in my life for my own personal computing device since March 2011 when I came home from a USA trip with 2 iPad 2's, one for me and one for the wife. They are both still in use as our kids each have one of them now. Gaming, light personal level productivity like budgeting and vacation planning, family calendar and schedules of events, personal banking, email, photos, home videos, social media, Christmas and birthday cards to relatives, the household's music and movie/TV show library consumption, it is the perfect device IMHO. iPhone is nice to just take out of your front pocket and do a quick thing or two with while on the move, but iPad is just better in almost every way for all of the things I mentioned above when on the couch, in the bed, in a plane or train seat, etc.
So - MB for my professional device, iPad mini 3 for my personal device, and iPhone 6 as the ultra portable for both personal and professional use...and as my phone and go to camera. For me, it is the ideal combination.
Obviously not everyone will feel the same!