Game development is hard and making anything substantial will require spending thousands of hours in either engine, so it's more about the purpose of the engine rather than ease of use, since you will eventually have to learn all the ins and outs of either one.

Unreal is a "AAA" engine originally meant for large teams. It has much better level design tools and collaboration features, but all programming must be done in C++ which is fragile and can break the whole engine. Lighter scripting can be done with Blueprints which is good for designers making levels interactive. Unreal is a good choice if you are aiming to make a serious production with a team making large detailed levels.

The Unity Asset Store has many more plugins and it's easier to put together a sandbox type game with lots of features, since its managed code design makes it easier to drop in scripts other people have made. There are level design tool plugins available that mimic what Unreal does, but they aren't as polished and collaboration is harder.