I too wrestled with this one. I ended up making 2 mock up's, and you can see from the attached images, the 6P is just a bit bigger.
Don't plan on a lot of savings size wise. Maybe 8% difference
That said, I'm not sure its my cup of tea coming from an Iphone 5S, but we'll see.
It does take some getting used to, and I still haven't actually carried it yet.
FYI, the 6P is under the cardboard on the left, under the 5x template. Left and lower edges are aligned flush, that's how much bigger, the 6P is.
Your mileage may vary
That will be the test is if you can actually carry it around for work.