It really does depend on who you are and what you want to get out of your trip.

And again, you've offered no dates in September, so I've no idea where there might be a festival on during your stay. If it's between the 18th and 24th then I'd suggest a few days in Tarragona for the Santa Tecla festival.

Goodness knows I love Granada, but nothing trumps the experience of a great fiesta as far as I'm concerned. Tarragona is located between Barcelona and Valencia and has excellent links with both, particularly when using the high speed Euromed trains. It's also a lovely destination in its own right with a highly visible medieval heritage.