Personally, I'd stick with Barcelona. Your seven nights is just six full sightseeing days. Even if you spend only 2 nights in Madrid, you're reducing your full days in Barcelona to only three. Or you could go straight to Madrid on your arrival day and spend 3 nights there, but you'd still have only 3 full days for Barcelona. That's going to prove frustrating when you try to schedule your visits to the many sights for which you'll be buying tickets in advance. And the weather will be milder in Barcelobs.
Although Barcelona doesn't have the Prado, the MNAC is quite good, the Miro Museum is a joy to visit, and as art fans I assume you'll want to brave the utter madness that is the Picasso Museum. And there are other art museums.
Southern France is lovely, but I don't think a 2- or 3-day trip from Barcelona would be smart. Tnere are lots of nice art-related sights, but they're scattered all over the place, not usually terribly near the Spanish border, and they are often located one per town (except for the over-endowed Nice). That area deserves its own trip, for sure. Also, Google "mistral".
