Last April we did exactly what acraven and david suggests. We took advantage of TAP's Portugal Stopover Program and we stopped over in Lisbon for 5 nights on our way home from visiting our daughter and her family in Marseille. We have not been to Porto so I can't compare, but we LOVED Lisbon. We spent about 3.5 days in Lisbon, and one day in Sintra and Cascais. We don't usually do bus tours, but we were tired by the end of our trip, so we did take a bus tour to Sintra, which included a brief stop in Cascais.
There is so much to do in Lisbon. I suggest getting a guidebook. Some of the things we did: Gulbenkian Museum, Coach Musem, Sao Jorge Castle, wander around the Alfama, Jeronimos Monastery, went to top of Discoveries Museum, wander around other neighborhoods and visit miradouros (viewpoints), and a few small churches. Don't remember the names. One is in or near the Alfama, and the adjacent monastery has beautiful tile work.
There are other smaller museums that we didn't have time for, such as the Tile Museum.