Costa Brava or San Sebastian

The Costa Brava offers many opportunities for day trips to interesting towns, e.g., Empuries, Cadaques, and Girona. San Sebastián underwhelmed me.

I thought San Sebastian was a worthwhile stop (I spent about a full day, or a bit more, sightseeing there) but preferred Bilbao. I'd want to see both cities if I traveled in that direction. There are also lots of pretty little coastal villages, and the Basque countryside is lovely. But it takes much longer to get there than to somewhere like Cadaques. Check out the (allegedly optimistic) driving times on viamichelin.com; that may help you with your decision.

I'm a huge fan of Gorona, too.

Both are good.
So you finish up in Barcelona?
Where do you fly home from?

We will fly home from Barcelona if we do Costa Brava, fly from Bilboa to London if we go to San Sebastian.

I think you'll probably get more from the time available by staying on the ground in Catalunya.
Visit the Costa Brava and fly home from BCN.
•Girona, Besalu, Figueres
•St Feliu de Guixols, Llafranc
•Tamariu, Begur, Aiguablava
•Empuries, Cadaques
•Port Lligat, Port de la Selva, Sant Pere de Rodes