Advice on choosing between two cities
 

Hello there
Two friends and I are planning to travel Europe for a month in May. We have most of our itinerary planned out but are stuck between two places for our last destination. Edinburgh and San Sebastian. I know they couldn't be anymore different so the decision is difficult, but any clarity would be fantastic. We are 20 years old and are looking for fun, high energy places but at the same time want to cross those cities of our bucket lists that have amazed us for so many years.
Here is what intrigues us about each:
San Sebastian
we would love to go surfing while in Europe and have learned that the waves here are pretty great. The city center looks extremely interesting and its proximity to a fantastic beach excites us. Pinxtos looks amazing and we have heard the nightlife is quite good. My only worry is that we are also visiting corfu, Ibiza and Barcelona, all being major beach areas I don't want our trip to kind of blur into one.
Edinburgh
We went to Ireland on a rugby tour a few years ago and fell in love and Edinburgh, from what Ive heard, has somewhat of a similar vibe. I am in love with cobble stone and castles so not only does the city interest me but to get into the highlands as well would be a delight. It would add some diversity to our trip. We are flying into London so it is not too far off our route. My two friends worry that there isn't much to entertain us in the city. I feel the opposite.

If anyone could shed any light on the two destinations that would be great. Personal experiences, or research you have done would be extremely helpful.

Thank you all
Alex

As much as I like Edinburgh, it doesn't compare with Donostia-San Sebastian on the culinary level unless you actually like Hagus (Haggis). The weather will be milder and the sky a little bluer, but still too early for beach weather.

We will be coming through San Sebastian in early June. Will that be better weather?

Variable...no one here can predict the weather. Someone can maybe give a link to a weather history site, but that is also no crystal ball.

May/June is definitely normally beach weather in San Sebastián, and without a doubt if you are planning on surfing. I shared an apartment with some French guys here some years ago, and they were surfing most days of the week from early February until they left in June. The city beach Zurriola is where to surf. Also great surfing in close by Zarautz just west of San Sebastián.

In late May/early June you can expect lots of sun (past 17 years statistics says average 7 hours/day), lots of rain (not uncommon with quite heavy showers almost every day) and temeperatures at 70+, often into the 80's, and every now and then a chilly day.

Great walks along the rugged and dramatic coast and into the close by western Pyrenees.

I would do Edinburgh - love it and the entire area. If you head up from London you can stop in York to see Viking area remains, near the border to see Roman ruins and exhibits and a ton of stuff in edinburgh (yes, castles etc) and also some of the highlands.

However, I'm not a surfer. And while I like San Sebastian I think 3 or 4 days there is enough. From London up to and through Scotland you can easily spend weeks.

It's impossible to choose between two such diverse places. When you ask "which one" you're going to get the opinion of the responder who doesn't have the same interests as you do. Choose the one that offers you more of what you want from the destination.