Difficult one really. As we were only in Spain for 3 days on a weekend break it was only ever going to be a day trip. For us just strolling around soaking up a place is more important than finding out how many arches a Cathederal has, so as its quite compact, we felt we had "seen" it all in the one day. That said, everyone says its a better place at night when the tourists have gone ( but youre still a tourist even if you stay overnight!!)and it does look like it would be very atmospheric in the dark. Overall, if youve got the time, I'd stay overnight for that experience rather than any extra sight seeing time. Hope that helps.