The 6 Plus possibly as it has a low res screen to power. Still the battery size itself is tiny to what it should be, I would say the two devices may be even? But you have to think about background tasks and location based apps.
Apple don't consider battery life as important.
But to be honest mate, if you want to use your phone for music and you're heavily invested in the iTunes store then just get a 6 Plus and try it.
If you don't like it return it. They are both the size of houses haha, but the 6 Plus is the smaller device, just, also the 6 Plus I think would have the better camera?
