To summarize the evidence so far: we have an article from a respected source, echoing Senior Product Marketing Manager Marc Diana in saying that PS4 implements their hUMA architecture while the XB1 does not. This is followed by the deduction that PS4 will have a significant performance boost over XB1 because of that. It remains unclear what the AMD representative said verbatim. We can only say that c't is as reputable as it gets in the space of German IT publications; that does not make them infallible, of course.
In addition, Sony is part of the HSA consortium while Microsoft is not. Of course, we don't know the political implications of such a membership which may play a significant role along purely technological reasons.
Technology-wise, we can say with certainty that PS4 indeed is a hUMA architecture. Everything we have heard since the release from official sources, especially PS4's Onion/Garlic memory bus layout and the volatile tag on cache lines, confirms that. In addition, Sony has actually advertised this fact quite aggressively, beginning with the Havok GPGPU demo at the PS4 reveal.
We don't know much what the Xbox One does beyond what the leaked documents tell us. And those are ambiguous and not very detailed. At first, it sounds unexpected for it to not support hUMA. Microsoft certainly has implemented a more custom memory system than Sony to manage its DDR3/ESRAM layout, including its DMEs. There is no clear indication about which features of a hUMA architecture might be lacking. Maybe not all memory clients and pools are cache-coherent system wide. The GPU does seem to have to flush its caches when it synchronizes with other clients. However, the article on vgleaks is neither very detailed nor clear, nor do we know if its author recited the original sources correctly. So the relationship between a hUMA feature set and the XB1 is unclear.
Politics-wise, AMD certainly wants to push hUMA since it is part of its own upcoming end-customer products. This would be reason alone to champion the PS4 independent of what the XB1 does, since AMD won't profit from what Microsoft has done with its memory subsystem.
Apart from that, developers at gamescom seem to acknowledge anonymously that the PS4 has a clear 3D-performance advantage over the Xbox One which is certainly a result of many factors, not least because of its beefier GPU and its straight-forward, high-bandwidth GDDR5 setup.
