This July we are on a cruise that stops in Cadiz for a for about 8 hours. We have an option to visit Seville, but would be limited to 3 1/2 hours. Explore Cadiz or Seville?
I haven't been to Cadiz so I can't really compare, but you could easily see the Real Alcazar and the cathedral in Seville in that timeframe.
Cadiz was a disappointment to me. It's really run down and across the water from a busy port. I'd take Seville, see the Alcazar and Cathedral - which will easily consume your time.
Another option, just up the road between Cadiz and Seville is Jerez. It's the Sherry capitol of Spain. The Bodegas are the only ones in Spain that allow drop in visits from the public. It would take less time to get there so you could visit the Tio Pepe (Domicq?) Bodega and nearby Osborne during your time. Jerez was a pleasant surprise for us, at least worth considering.
I would go to Seville and try to get on a 2 hour walking tour with Conception. Then I would hit a couple of the tapas places she suggest on the tour or whatever else that may interest you and she can point out. We just got back from Spain last night.
Sevilla ... check out Really Discover Sevilla Tours....small, and excellent!!!
Thank you all for your suggestions.