I strongly considered doing this for a year becomes the 6 Plus was huge and the iPad had no real differentiator aside from a nice larger screen. However, I never liked the Plus' size for on the go use.
All this changed with iPad Pro and iOS 9. I take my device with a keyboard (now an ASK) to class to take notes, which is not acceptable with the iphone, I use split view and (most importantly) have the Apple Pencil.
Now the iPad has differentiated itself and can do things the iPhone can't. Now I have no issues justifying it and with an iPad for semi-mobile use, I went with an iPhone SE for super mobility and ease of use.