There's an elephant in the room comparing the two. A month ago, Epic (Unreal Engine) acquired Quixel Megascans - a company which makes photorealistis materials.
This scans are going to be included in for free in UE4. This is a huge deal since all developers will have access to 100,000+ sleek assets from the start.
I've heard it from Unity expert themselves telling you can achieve the same quality (that's why I've included the performance and graphic comparison).
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?
Elitists club on the way. Actually from the business perspective that's a smart move.
I'd love to know what are the churn numbers of Amazon Prime program.