Ok, sorry if I might be repeating something someone else said in last 12 pages (I'm catching up on 11th right now...), but to me it seems like:
Microsoft explicitly wanted 8GB of memory, to ensue there's room for all the non-gaming stuff (Kinect, TV, Skype, Apps, etc). As a result of fixed price point, they needed to go DDR3 route + embedded RAM (to compensate for lower b/w), which - best to my understanding - excludes full hUMA compatibility. To make up for that shortcoming, namely to make the copying of memory between GPU and CPU pools faster and 'cheaper' (in terms of CPU performance) they've added the Move Engines.
Sony on the other hand wanted unified memory (with high b/w), so they chose GDDR5, even if that meant they'll only have 4GB of it. Still, that setup allowed for full hUMA compliance and was easy to develop for. Sony got lucky 8GB was possibly by the launch time and only now are adding stuff to the OS, to rival MS's non-gaming features (that's my impression at least).
In the end I expect both Xbone and PS4 to perform similarly in that regard, i.e. memory operations for GPGPU (I'm not touching the 1.8TF vs. 1.3TF GPU difference here, additional CUs, ROPs, ACEs, etc), only they've chosen different routes to achieve that. It looks like MS's memory setup is more convoluted, but I'm sure their SDK will hide a lot of it for the devs to not worry about.
So, my interpretation is that PS4 is "full hUMA", but Xbone is a "hUMA derivative", in a sense that it can achieve similar results (again, talking only about memory) with similar performance. If that's the case, no wonder AMD will hype Sony's solution, because this is something they can sell to PC market - a clean, elegant and efficient solution. Even if MS's approach will yield similar results, it looks much more complicated and most likely is more expensive and definitely more difficult to manufacture (i.e. all the APU yields rumors).