Coming from a 6, I had no problem springing for the 6S Plus and haven't regretted a single second with it. Best phone I've ever owned and I've had every model of iPhone since the 3G.
My iPad Air 2 w/ Wifi only is used for school, eBooks (tons of them), full-screen productivity apps (iWorks, Codec, Evernote), and media/games that look better on an even bigger screen. There are plenty of them out there.
I would never trade off my Plus for my iPad because each gadget has its use and functionality. JMO.