It's like estuary (Bilbao) versus ocean (San Sebastián). Bilbao has the more interesting architecture and city layout, plus it has the iconic Guggenheim Museum. It's the larger of the two. San Sebastián has the ocean and the beach and its famous surfing. It's smaller, and it feels that way.
From what I can tell, for most people, the ocean wins out. No one that I can recall has ever told me how great Bilbao is. But many friends, old and new, from inside Spain and out, have told me of either their love for San Sebastián or that it was a place they were told they simply needed to visit.
And for me? I don't know. The ocean is amazing. But I'm a mountain boy, and Bilbao is nestled on both sides by mountains.
Understandable. I grew up in San Diego, so there is a chance that living in a coastal city made me more inclined to like San Sebastian.
Now that I think of it, I’ve actually never been to Bilbao. I’ll have to give it a try if I’m ever able to go back to Spain.