Are you planning on flying from Italy to your next destination, before heading home? Are the arival/departure times and prices better in one place or the other? Lisbon is, obviously, farther from Italy, so will likely take a bit longer to get to. Either one gets you to the edge of the ocean.
In Lisbon, you can have salted cod fixed 20 different ways (all good), listen to fado music, and take interesting side trips to Sintra and Belem. Portuguese beer is stronger than its Spanish counterpart. In a 2002 trip, the Rick Steves-recommended place we stayed at, 8 floors of stairs up, was definitely NOT fancy, but it was the best deal for us at the time. You'll enjoy the ornate black and white cobbled sidewalks, hilly streets, and funky tram cars.
In Barcelona, developed in-part after the olympic games a while back, you'll have a big-time big city, with interesting old and modern parts, great Picasso and Miro museums, the awesome and still-under-construction Sagrada Familia church, and other Modernista architectural creations by Gaudi and his compatriots. Spaniards (including the Catalunyans in Barcelona) stay up later, and stay out later than many, if you actually were looking for nightlife.