I have a hero 5 black and Karma Grip. I bought the OSMO because I'm thinking this will be a good everyday camera and the GoPro will be for me outdoor adventures.

I'm not a video pro I just make videos for myself and family.

Question is the following.

1) to the naked eye is 4K, 2.7 and 1080P noticeably different?

If not which option which give more battery life to the OSMO and the iPhone 6S+?

"1) to the naked eye is 4K, 2.7 and 1080P noticeably different?"

Depends.

It all depends on the size and resolution of the device you're watching it on.

If it's your cell phone -- probably not. If it's a 27" 5K iMac -- then definitely yes.