Biarritz or St Jean de Luz or neither

Option D: Have you considered going to Espelette, a typical Basque village 30 minutes from Saint-Jean-de-Luz by bus?

https://www.en-pays-basque.fr/en/territory-and-destination/nivelle-valley/espelette/

There you'll find unique peppers with a protected designation of origin, sometimes used in French cuisine.

Or option E - couple of nights in Bayonne, much nicer than Biarritz. Small, charming town with two rivers, old streets and interesting shops, art gallery and musuems, nice bars and restaurants.

Add Ainhoa to JoLui’s suggestion of Espelette— they are two beautiful Basque villages.

And I agree with Lin C that Bayonne is better than Biarritz.

I agree with the recommendation for Bayonne, which is a lovely town. The Basque History in Bayonne is wonderful.

Another vote here for Bayonne over Biarritz or SJDL.

SJDL and Espellete are both lovely for a short visit (a few hours or half a day) but not for a couple of nights. And I wasn't impressed by Biarritz at all.

My vote is for SJDL.
It's a quaint little seaside down that is worth visiting.
Sure, if all you are doing is visiting and not staying there, then one afternoon is fine.
But if you actually stay there and walk the beach, enjoy the restaurants, and just relax, you will love it.
We stayed two nights, three days there before our RS Basque tour that started in Bayonne.
We stayed at the Grand Hotel Spa and Thalasso right on the beach with ocean view rooms. It also had a spa and swimming pool.
https://luzgrandhotel.fr/en/
It was a great way to get over the jet lag. we walked the beach, ate at the local restaurants and window shopped at the local stores.
We didn't eat breakfast at the hotel because it was too expensive, so we walked the Grand Plage (their boardwalk) along the beach and found one of the local restaurants that were open for breakfast. we used Yelp to find the better restaurants there.
we Walked into the town and saw the sites.
Walked out to the promontory point at the north end of the beach
And we ate at wonderful French restaurants. (which don't open until almost 7 or 8 pm.) And there were a lot to choose from. The seafood was amazing.

The one down side? I remember that despite taking 3 years of Spanish. I had to wait until we crossed pyrenness to speak it. They speak French or Basque and I had to pull my high school French out of my ...... back of my brain.
My big faux pas was NOT greeting everyone with "Bonjour" before I tried to interact with the locals.
Once I figured out what I was doing wrong and corrected myself, the locals were warm and friendly and helpful.
But don't worry. In the tourist areas, they speak some English.

In comparison, Biarritz is just high rise buildings along a beachfront. No charm whatsoever.

I love SJdL, but Bayonna is great too, and I agree that it would be the better place to stay for a few days, especially in May. (Not beach weather).

What I´d like to add is the suggestion of cutting time from San Sebastian and spending it in Bilbao. San Sebastian is nice, but there really isn´t as much to see, while Bilbao has much more to see and do. Also, having visited the French coastal area, San Sebastian is very similar. If you do a search of the forum, there are several threads about Bilbao vs San Sebastian.

I like both of these towns for different reasons as well as Bayonne. SJDL is more of a low-key beach vibe, Biarritz is bigger, more touristy but both offer great beaches (if that's important to you.) Bayonne is full of charm and is a wonderful Basque city. You also might look at Hondarribia, which is a seaside Basque town just over the Spanish boarder. Super charming! As for San Sebastian, we love it and go once a year. Two nights is good unless there is some event you are planning to attend (concert, football match, etc.) I hope you enjoy your trip whatever you choose!

I’ll add to the chorus of those preferring Bayonne to Biarritz. As I recall, it has a small, but good museum of Basque history. Esplette is also a charming town.

My memory is fuzzy on this, but I think the town of Cambo-les-bains is also close by. It has the lovely basque style mansion and gardens of author Edmund Rostand (Cyrano de Bergerac and other works).