There’s no one answer, it really depends on the nature of the assets you’re creating and where you intend for them to be used.
3D Tiles excels at large assets, especially if they’re too big to copy to every end-user system so they need to be streamed from a server. It also has a solid approach to managing level-of-detail so that the rendering itself doesn’t become overloaded.
On the other hand, regular Unreal Engine assets have more flexibility as far as fancy rendering effects, physics, pre-baked lighting, etc. At least today! This may not always be true.
I suspect that most Cesium for Unreal applications will benefit from using a mix of 3D Tiles and Unreal Engine assets.