The S5 does seem to have an upper hand on the front-facing camera, and in-door white balance is better.
The S5 video image is a lot sharper and more detailed with very vibrant colors. Whether those colors are more accurate or not I don't know, but it does look better in my opinion. Autofocus on the Z2 seems to be very good. The Z2 overheating after just a few minutes of 4K recording is slightly worrying, though. Makes 4K recording a bit of a useless gimmick on that phone. And, yes, lack of 4K displays and all that; it's still a prominent feature on that phone and should work properly.
Lay down your soul to the Gods rock n roll!