I've been to both.In Galicia and you really need to hire a car. Santiago De Compostella is a lovely city. Great traditional restaurant called "El Gato Negro" on Rua La Raina. Go to see Fisterra (world's end) and pop down to Vigo. Some excellent coastal scenery. Asturias is also worth a visit. Oviedo, in particular is a nice city. Gijon has a very good beach
I was in both San Sebastian and Bilbao last summer. S.S. is a beautiful city but more expensive and less friendly than Bilbao. Playa de la Concha is a must for a swim.Basques love eating so the ""pintxos" in the bars are excellent. In Plaza Nueva in Bilbao there is a bar called Café Bilbao. Superb, reasonably priced pintxos. Cracking social life in Bilbao, friendly people a lot of whom speak English. You won't get a bad reaction if you speak in Spanish. I didn't hear one person spaeking in Euskera while I was in Bilbao. I was answered in English when I enquired in Spanish on more than one occasion in S.S. You should also visit Portugalete and San Juan De Gaztelu outside Bilbao. Very good public transport system of trains and busses from Bilbao. Car not really necessary.
I was there in August and the weather was generally very hot. However, being on the Northern coast, rain is never too far away Dog!