if you are building for mobile platforms and you are indie , or your game is relatively small , stay with Unity .
Dont get me wrong , not that unreal cant do it , it just doesnt comes out small ! an empty project built for android is 60+ MB ! for unity it is 15 MB , if you build for android arm7 , it’ll be 7 MB . in cocos2d-x it is 1 MB.
So unless you are planning to make something big , like Lords of the fallen< android version> which is about 1.3 gigabyte in size <made in unreal> , stay with unity for mobile .