I have used both.
Instance i would use Unity: 2D and mobile apps. Also, some linear “low-poly” 3D games (not large scale)
Instance i would use Unreal: 3D, open/large world, heavy sided graphics (“today-gen” graphics, higher poly, more PC oriented)
Unity has a better support system. Been around a little longer. Community is cough nicer cough lol. More assets. More choices.
Unreal has a better engine (in my opinion) for what i am doing, so, i am here. Unreal seems to be catching up. Its like asking “whats better, PC or Mac?” Really depends what your doing with it.