This is a very special question, and I don't know if any of you Kowa users around here have tried both adapters/cameras.
I have used the DA10/4 system, first with my Canon Ixus, later with a Sony RX 100 II with external viewfinder and USB shutter release, for the last six years. The DA4 is a well made adapter, but imho the camera mount on the its plate bears some risks for digiscoping: even if you fasten the screw, the whole system is not stable and the camera may move just a tiny bit horizontally when you hinge the adapter aside for a direct view. Even fractions of a mm will mean the camera lens and the eyepiece of the Kowa TE11 eyepiece are off-axis which may result in focussing problems. All in all, you always have to control everything very thoroughly to achieve the best results.
Furthermore, as we RX 100 users will know, you always have to hinge the camera aside when turning it on or off because the lens moves out of the camera body so far that it will touch the eyepiece. Even if you adjust it after bringing the camera back into position, there is always a gap between the eyepiece and the camera lens which will allow light falling in from all sides.
Of course you can find the sweet spot where the two glass faces (eyepiece and lens) almost touch each other and remember it (it will be between zoom 2,7x and 3x on my RX 100). But you will have to adust it each time after you turn the RX 100 off and on again.
The Kowa TSN-IP 6 looks like a much simpler solution: you just shove the ring over the eyepiece. The lens of the iPhone will be fixed by the frame of the adapter, and the ring makes sure there is no shift concerning the axis. You can use your iPhone ear plugs for shutter release to avoid a shake of the system. The photographic apps for the iPhone (e.g. procamera) offer a lot of surprising parameters such as RAW (not for iPhone 6, though) and a very sophisticated and fast HDR.
Have you used both adapters, and which one do you prefer nowadays? I would buy the Kowa TSN- IP 6 but want to be sure it will be an improvement.