If it were me, I would choose the city based on what sight is most important to me: the Alhambra in Granada or the Mezquita in Cordoba. While I loved the Mezquita, the Alhambra, for me, was more amazing and more stunning than the Mezquita. The beauty of the Nasrid Palace is beyond words. We spent about 3/4 of a day at the Alhambra including the Nasrid Palace, the Generalife Gardens, the Alcazaba and having lunch there as well. Then we returned in the evening for the magical night-time tour of the Alhambra. If you choose Granada, I would definitely spend at least one night here; two would be better. It really is too far from Seville to make it a comfortable and enjoyable daytrip.
The advantage to Cordoba is that it is closer to Seville so can be done more easily as a daytrip. Don't get me wrong. I loved both the Mezquita and Cordoba, but if I had to choose one, it would be Granada to see the Alhambra.