I'd go Nexus 5 easily. I think it's a nicer looking phone, has a great soft touch back and rubberized feeling sides (only on the Black model though), and the front glass has no markings. Also the Nexus 5 has a faster Snapdragon 800 processor. That along with stock Android will make the Nexus 5 feel much snappier. Samsung is doing a good job so far updating the GS4, but I suspect it's update cycle is probably near it's end, while the Nexus 5 will enjoy at least another year or more of updates likely.
PS. For the sake of balance... some things the GS4 has going for it... The Nexus 5 battery life is not that great, while the GS4 has a removable battery. Nexus 5 is limited to 16 or 32GB, you can expanded the GS4 with micro SD if you like to carry your own large music library or movies around. The AMOLED display is pretty nice.
Still, Nexus 5 would be my choice definitely.