My husband and I will spend a week in Paris (where we have been many times) and then want to spend about 5 days in the Basque region (where we have never been). Definitely want to visit both Biarritz and San Sebastian, but where would be the best place to be based? (Want to relax on beach, but also want to do a day trip to Bilbao.) Thanks very much for any advice.
I liked Bayonne much more than Biarritz and Bilbao much more than San Sebastian, but I admit the beach in San Sebastian looks lovely. I think the Spanish Basque Country has more to see than the French (though the latter is interesting, too), so I guess I'd go with San Sebastian of your two choices. SS is well-positioned for side trips across the French border as well as to Hondarribia, Zarautz/Getaria/Zumaia and of course Bilbao (the frequent buses are faster than the trains).
You should be cautious about your expectations for time on the beach, if that includes getting wet, because the weather in the Basque Country is unpredictable and is very often overcast when it isn't raining. When I traveled through that area in June 2016, just about everyone I saw in the water (which was very few people) was wearing a wetsuit.
San Sebastian has a nice beach but no one was there in mid July when I visited because the weather was kind of chilly rainy humid.
A day trip to Bilbao would be a total slog from Biarritz. Also, there isn't that much to see in Biarritz, and the main beachfront is marred by unfathomably ugly development. I still do really like the vibe of Biarritz, but my family is partly local and I am thus very biased.
So, I also suggest San Sebastián.