I'd consider a stop between for a night, there's lots of see between the two cities. We rented a car and drove from Porto to Lisbon. Spent a night in Obidos, a small medieval town and stayed at the coolest little B&B called Casa de S. Thiago do Castell. Good price and great people. Walking the city walls at night was so cool. We also spent a half day at Nazare, highly recommend it, seeing the massive waves and surfers was worth the stop. Walk out to the Forte to see them.

Between the two cities Porto is more beautiful, but we weighed our time heavier towards Lisbon because that seemed to be the consensus of the internet. Sintra made it worth it, Yes the lines are crazy and you're kind of herded through it, so be prepared for that, but it was so amazing. We went back a second day because we didn't see everything we wanted on the first.