You have only six full, non-jetlagged days in Spain. The arrival day will almost certainly find some of you sleep-deprived and struggling to stay awake; for some of us, it's all we can do to remain vertical. So I agree with Carlos; stick to the cities you are flying into and out of. For your next trip you can concentrate on Andalucía (but you should try for a few more days). Seville is lovely, but would require 5 hours on a train round-trip, plus travel time between your hotel and the train station, and more time to between the station in Seville and the historic center. And the city has several days' worth of sights.
As for splitting your time between Madrid and Barcelona, I think a lot depends on how interested you are in art, because Madrid's major points of interest--other than the Palacio Real (which many people like but I did not)--are the three big art museums. If you don't plan much art-museum time, I suggest no more than 3 nights in Madrid unless you are prioritizing a day-trip to Toledo or Segovia. Those are both great places, but you have so little time that I wouldn't want to spend it on going to other cities. On the other hand, with zero interest in art, you might consider making this a Toledo + Barcelona trip. It would be very easy to get to Toledo on your arrival day.
Weather-wise, Barcelona is likely to be somewhat warmer than Madrid. The average highs and lows are about 10 degrees (F) higher in Barcelona.
There are a bunch of sights in Barcelona for which you'll need to buy tickets in advance (if you wish to see them), because you will otherwise almost certainly face long ticket lines and possibly sell-outs. A lot of people like to go to Europe over the holidays, and Barcelona is very busy year-round. The problem sights are: La Sagrada Familia, Parc Guell, Casa Batllo, Casa Mila/La Pedrera, Picasso Museum, and possibly the Palau de la Musica Catalana (limited number of English tours).
Note that the Picasso Museum can be horribly crowded to the point that it is very difficult to appreciate the art. I don't recommend it except for people who are big fans. This is not to suggest that the other places I've mentioned are uncrowded--quite the contrary. But it's easier to appreciate architecture in a crowded building than art.
