Both are great engines but UE4 is out of the box much better then unity in terms of systems like camera/lighting/physics ease of use. Unity is slightly behind but can still make games and movies as well and sometimes even better then UE4. It all comes down to how much time and resources you want to spend into engine coding. To bring Unity on par with UE4 you'd have to devote time working on the core engine . A good example is Rust on Unity, it looks great now and is smooth but they have spent more time working on the engine than the actual core game mechanics. They had to re-write entire engine mechanics to get what they want, In UE4 you may have to re-write some parts to work as you want but it's far less extensive. The ease of use is the biggest part, In UE4 you can go from idea to finished asset in about an hour and Unity from my testing in about 2-3 hours. The time saved by UE4 is the important part. My only real problem with UE4 is mod support or the lack of it, Unity hands down wins mod support in games that allow them like rust. I would hope Ember does allow mod/plugin support as Firefall did to an extent.