6 -> 6 Plus. Never had any regrets and it was easy to get used to the bigger size. Actually, let me bring up my smartphone-past to illustrate just how easy I have been able to adjust to various screen sizes:
iPhone 5 (4") -> Lumia 1520 (6") -> iPhone 4S (3.5") -> iPhone 6 (4.7") -> iPhone 6 Plus (5.5")
Mind you, I never switched because for screen size until the iPhone 6 Plus and I've been happy with every size I had. Sure, the iPhone 4S felt really small after the 1520, but after a few weeks you stop noticing it.
I probably could go back to the iPhone SE's size, but I don't intend to. Much better viewing experience on the Plus.
iPhone 5 (4") -> Lumia 1520 (6") -> iPhone 4S (3.5") -> iPhone 6 (4.7") -> iPhone 6 Plus (5.5")
Mind you, I never switched because for screen size until the iPhone 6 Plus and I've been happy with every size I had. Sure, the iPhone 4S felt really small after the 1520, but after a few weeks you stop noticing it.
I probably could go back to the iPhone SE's size, but I don't intend to. Much better viewing experience on the Plus.