Almost without question I'd choose Unity for most projects. Its rich ecosystem, large community, and decent documentation make it a solid choice. Many of its warts like only supporting an ancient version of C# have been remedied in recent releases (as of Unity 2018.3 you can write C# 7.3, which is a nice language indeed). Unreal's Blueprints are slick for gameplay scripting, but I'd choose C# over C++ for game code any day.

In the end they're both excellent engines, and I suspect a AAA game would be better served by Unreal, but I find Unity far more flexible for arbitrary game designs whereas with Unreal you sometimes feel like you're fighting its roots as an FPS engine (though if you're making an FPS, UE4 might be just what you need).