The Wi-Fi would be more convenient to have on a phone but after thinking about it I would have to go for the phone that's always fully charged. Why? Because what I would have would be more than just a convenience, it would be a device that somehow produces infinite and 100% environmentally friendly electricity. If we could harness that unlimited resource, perhaps even find a way to mass produce it, we would have an unlimited supply of clean and free electricity. It could be the basis of a new technological revolution, never have to worry about producing power and the pollution involved with it again and we could provide electricity to everyone in the world. It would improve the lives of everyone on earth and even allow us to move beyond it. Hell this is ignoring the benefits to myself, a discovery like this would probably make me insanely rich. Even if I never made a cent it would still be worth it.
So yeah, I'll take the phone that's always fully charged.