All things being equal (and I know they're not), I think your southern route would add some diversity to your trip. Is this your first time to Spain? I was fascinated by Granada, Cordoba and Sevilla. Those places are what was in my mind when I thought of Spain (before seeing more of the country). Don't get me wrong, Basque Country is one of my favorite places in the world. There are beautiful beaches, great restaurants and green mountains. But everyone should see southern Spain, even in the heat. It's so different from Barcelona. So, although both side trips are great options, I vote for the southern Spain route - especially if this is your first time to Spain or if you won't be returning again soon.