Well, I've used both extensively, and they are good at different things. Unreal is far better IF you fit cleanly into one of their templates (first person, third person, etc). Superior renderer with great built-in post effects, FAR better multiplayer support, better materials/shader editor. However, if you try to do something substantially different than they have imagined, it can get ugly very fast. The C++ used is strange, with all kinds of weird GC macros strewn about. As a programmer, I hate blueprints. I wish there was a scripting language alternative. And god help you if you actually want to modify the engine code substantially, it's a monster (not poorly done, just sprawling and extremely complex). Look at the nvidia gameworks integration, it's all these forks of the engine built by random people, it's a mess.

Unity shines in its flexibility, easy scripting, and fast workflow. I can whip up a script in C# very quickly that does exactly what I want. With that scripting, I can have a lot of control how the engine and rendering and post effects work, much more than Unreal. Prefab workflow makes much more sense than anything in Unreal blueprints. Mecanim is not as fully featured in some ways as Unreal's animation system, but with the Humanoid skeleton, animations can be used easily on different models. With Unreal, you need to use retargeting, which is painful and can have serious problems if the models are even slightly different in size.