It's shiny, basically.
In terms of pure speed and ease of development, even ignoring the relatively painful difficulty curve in UE4 I've mostly conquered by now, you can't beat Unity. I'm probably anywhere from 25 to 50% slower at any given task in UE4 (except maybe character animation and particles, which are comparable) when compared to that same task in Unity.
Still, I'm absolutely confident that I could never match the fidelity I've got in my prototype right now with a Unity build. I reluctantly admit that it's technically possible (Unity puts out very impressive demos all the time), but not to someone with my skill level, I think. I've very happy with what I've got so far.
The 5% might sting in the end, but I'm trying to catch some eyeballs!
Thanks, yeah I'm pretty deep into Unity now and keep looking at UE4 out of the corner of my eye... partly due to the new and shiny, and partly due to the overhead that Unity carries with it.