Just watch any videos on youtube. Iphone 6 plus is excellent. Great colours and range (it looks almost semi hdr) Excellent stabilisation, really good detail without having to record 4k then downscale it. The normal iphone 6 is really good too, but the +'s optical stabilization just makes the stabilisation that little bit better.
The Z2 has 2 levels of digital stabilization. The 2nd level can only be used in HD which isn't that detailed. 4k can be taken with the first level, but If you compare its 4k to that of a samsung it is really soft. I think its the lens. Stills look soft as well. So even with 4k I dont think you are getting the detail of the iphone 6/6+ in HD.
The samsungs in 4k have more detail, but to me the image is too contrasty with no range and over sharpened.
The fact that you already use Iphone, Its a no brainder. Get the +