It depends on the situation
UE4 has a lot of features that Unity lacks, Unity by itself is almost unusable, but there’s lots of great things in the asset store to add the features you need to Unity. In some of the projects I’ve done, there’s a specific feature I need that is available in the Unity asset store that UE4 doesn’t have–like Windows multi-touch support. Stuff like that will improve over time as the UE4 marketplace grows.
Unity is great for developing for low-end systems, it starts out with very little graphical features so it’s easy to add what graphical features you want. Unreal starts out with a lot of graphical features which can be complicated to figure out what to disable or adjust to get things running well for low-end systems.
The nice graphical features all work well in UE4 though, the material system is especially great and the lighting system is easier to work with. I’ve actually avoided using Unity 5 because the new lighting system is worse than it was before.