[caribousoup]

Unity editor is more stable. Unity editor is more performant.

UE4 is easier to work with in the long run. I find that Unity is just missing stuff, or has half-baked solutions that change. UE4 already has this stuff included as core product.

Unity is more approachable, has better docs, but slowly falls apart. UE4 is less approachable, has worse docs, but becomes better to work with over time.

But, it all depends on your use case, and what you need out of which tool. I have worked in Unity 4/5, UE4, Source, and Source 2 as of recently with the HL:Alyx tools.

Unrelated side not: Source 2 tools are pretty slick! But they don't have general licensing yet... if they announce that, my next project might be in Source 2 instead of UE4. Something about that Quake/HL legacy code that is always more comfortable than Unreal.