I am bringing my son to Europe from September 14th to September 24th. I am just wondering if Barcelona or Athens would be better.

The full ten days in one city?

La Mercè Festival would be the last bit of those ten days.

Other than that all I can say is it depends on your interests.

Depends on your interests, and your son's! Both cities have much to offer.

How old is your son?

I agree with with kja and Traveler_Nick. We need to know what your and your son's interests are in order to give good advice.

Welcome to Fodors. We need more info like: what sorts of things do you want to see/do? Do you mean all the time in one city or did you really mean which is better Spain or Greece?? Any other details that would help us -- age of son? his interests? etc.

In my opinion, Athens hands down...

Barcelona is a massive, sprawling "modern" city laid out in the mid 1800's. While there is a small part of the old city that predates that era, most of what you see in Barcelona today is comparatively new, and rather bland. Sagrada Familia is a very interesting sight inside and out, and the easily accessible beach areas are great.

But in terms of wandering smaller streets, encountering great street food, small shops, and ancient ruins that tower over the city it's hard to beat Athens.

Athens is also massive and sprawling, but the inner/old city you're likely to visit is compact and easy/fun to navigate and explore on foot.
I have found the people in both cities to be friendly, but the greek seem to exude a more personal warmth.

“ But in terms of wandering smaller streets, encountering great street food, small shops, and ancient ruins that tower over the city it's hard to beat Athens.”
If I know teenagers, they are usually not drawn to the above…younger kids, even less.