As said, 14 days is a long time in either of those places
In Santorini, if you stay in Oia or Fira you will have access to many restaurants but you will have to travel to get to the beach. The red beach is quite nice, as is Perissa which also has a tonne of cool bars and restaurants along the beach.
In Barcelona there are ample restaurants but the beaches there are over-crowded with hoards of tourists and people trying to sell you stuff (someone will come up to you every few minutes)
If you go to Greece, look into visiting some other islands. Mykonos is lovely and Paros has the nicest beaches!
I have spent several days at each. Neither has beaches right on hand. At Barcelona there are good beaches an hour away by suburban train to north and south. At Santorini there is a smallish red sand beach on the south end of the island near the buried city, but don't expect 1/10 as many restaurants as BCN has.
I agree with CMV that 14 days is about 3x as much time as you need for Santorini (unless you just want a place to relax at). I would farther say that it's twice as much time as you need for Barcelona, even after throwing in Tarragona and the monastery in the mountains. Expand your trip to Crete or another island (for Santorini) or Madrid/the Pyrenees/Perpignan/Asturias (for Barcelona).
Oia. Santorini was my favorite vacation. Beaches have black sand, but water is warm. Lots of places to visit. Hope you take my advice!
The time you want to spend on either is far too long. Especially for Barcelona that beaches are not all that close around. Santorini in this case is a better option.
majorca
Oia is a village and even the Island capital is only a small town , in an island completely tourist orientated . You'll be climbing the walls there after 5 days unless you have a strong interest in wine production or in Geology / Vulcanology .
Barcelona is a big city and seaport but the beaches are a bit of a trek away by bus or tram . Barcelona has a lot more to visit , and you could spend a fortnight having a thorough look around and not see it all .
