Unreal has a better rendering system in my experience, and although this mostly matters for realism, it can also be very valuable in other situations.
One thing that maybe the "poisoning the well" element is the fact Unity had a generous freemium open doors for hobby developers, which means there's tons of project that are meh while Unreal Engine is well used in creation of AAA games.
Have you tried them both? Is Unreal really that much better for rendering?
Granted I've run into a few bugs in UE4, but then again it seems to include a lot more features out of the box. Lastly a lot of people seem to appreciate having the source code available which I don't believe is the case for Unity, as least last time I checked.
/r/Unity3D/ 159k members
/r/unrealengine/ 66.9k members