If you go via Biarritz be aware that you have to book a slot and the landing and parking charges are pretty eye watering too.
Cab from Biarritz to San Sebastion is about 80 - 100 euros if you hire a car make sure the hotel has a parking space or you'll be stuffed parking in SB is a nightmare.
However the place is brilliant if your into Pinchos Tapas etc you'll be in heaven.