it's both of them. good assets with good lighting. actually unreal can create much more beautiful scenes because it supports some techniques which they are not available in unity. the lighting model there is a HDR one and we don't have that in unity. also unreal has a visual material editor which helps it's users to create new materials easily without writing shaders. some of my friends think that it's lightmapping produce better results to cause after lightmapping all normalmaps and other stuff work properly but sometimes in unity you will lose them all when using lightmaps! but hey don't forget, unity is great!
to produce beautiful scenes you need professional quality assets and then you should carefully set the lighting. i am not a technical artist but even the most beautiful assets look stupid without good lighting and the best lighting with the highest resolution on mid-level quality asset doesn't look that good.