One day trip from Seville via train..Cordoba or Cadiz?

Hands down, Córdoba.

While Cádiz is nice to walk around it doesn’t have any wow sites, like Córdoba’s Mezquita.

I agree with jamie! Cordoba! The Mezquita is an amazing site, magnificent and historically important. Cordoba is a lovely city with its white-washed walls, narrow lanes, patios and flower pots. There are other interesting sights, too, such as the Alcazar gardens, the Roman Bridge, and the Palacio de Viana. Be sure to wander through the Juderia.

Agree, Cordoba.

Cadiz is lovely for hanging out, eating, walking, relaxing. But there aren’t many “sights.”

Cordoba is packed with many choices to see, especially the Mezquita. It’s a better choice for a day trip.

(I actually prefer Cadiz, but I’ve already seen the sights in Cordoba.)

Just returned to Barcelona from Cordoba, and to echo what others are saying here - it's a stunning city and really ideal for a day trip. If you can make it a long day trip, either arriving early-morning or leaving late-evening, all the better - especially at this time of year, it's a bit thronged midday at the Mezquita and in the patios of San Basilio. Very walkable and enjoyable for aimless strolling - in the spring, it's one of the best-smelling cities I've ever spent time in. (As a side note, if you're interested in the patios of San Basilio, bring coins - some of the patios are relaxed about asking for maintenance contributions, but others really want you to drop something in their box.)

Agree with all the above! Cadiz is nice but underwhelming. Cordoba is lovely and the Mezquita is truly awesome.

Thanks everyone...seems like a no brainer!!