Definitely, depends on what you want to do. Sevilla is more folkloric, Barcelona is more eclectic.
Barcelona is more european, Sevilla is more spanish.
Barcelona has Gaudi, Sevilla has a mixed moorish-christian style.
If you’ve been in the north you may as well try the south.
Good thing about Sevilla is that you can be in Cordoba in less than an hour and Granada in two and a half hours, so you get to see them too (Alhambra in Granada is one of the Seven Wonders).
And please, tell us what you finally did and how was it…