I am a noob, so my opinion might not matter much. I have only spent a week on trying out UE4 / Unity.
I have been wondering the same question the past week since both became more accessible and both free versions offer so much. I am making mobile game, so I spent some time getting something simplistic working, like custom camera that I would need. My basis of choosing unity was because it seemed to be more mobile friendly, and worked faster on my mobile phone. Until yesterday when I decided to let my friend test out something simple I made, the speed the game ran on my friends phone was quite unexpected compared to my nexus 5. The performance was just terribad, and I have been trying to revalue of things.
I personally have liked more Unreal Engines "openness". The debugging tools just feel phenomenal, not saying that unitys profiler is bad either, it is really good, but I like the fact that i can run console commands on mobile phone. Out of a box I feel like unreal offers a bit more than unity does, and the fact that you can actually export your assets out of unreal makes me feel more safe about it. Like if I made stuff with unity, there is no way to just possibly pull my project out of unity if I suddenly for example felt like source 2 would be the thing I really want to develop on. I have a strong feeling that in Source2 you can use C++. The truth is though, that I know that both of the engines are massive over kill for my small 3d game.
I like watching youtube videos for learning things, and unity has so much material in the internet. The bad thing about it is though, that most of it is out dated, although a lot of stuff has remained over the time, but a lot has changed as well.
Where I feel that unity works like charm is for importing stuff from blender to unity. With UE4 it seems bit more complicated to get working armatures from blender, but I am quite confident it will improve over time.
In my personal opinion somehow I feel to be more in control on UE4 than Unity. Also my gut feeling is that team collaboration works better in UE4 than Unity, but that probably doesn't matter to solo developers like us.
In the end it comes down to personal liking, for me my liking has started to lean more towards UE4 for some unexplained reason, although they both seem very capable for pretty much everything. Maybe my reason for going UE4 road is just having weird feeling of being a bit more safe with UE4, like the whole Free vs Pro with Unity has some possibility of being problematic for in the future, if I end up having a bigger team than going solo. I cannot really describe this fully with words.