Either, they're both great - friendly, small and easy to navigate on public transport, interesting architecture, lovely food, good value, probably more sightseeing in Lisbon but Porto has the Port houses for tastings
Both of them will be very enjoyable as long as you get decent weather; like most of Portugal they are miserable places in the rain.
In Lisbon the metro and tram system will get you virtually everywhere, get a '7 Colinas' travel card for unlimited use. You can also get the 'Lisboa Card' which gets you onto public transport and entry to many of the sights and museums, but you have to hammer throughout the sightseeing to make it worthwhile. For sights, a day in Belém by tram seeing the Jéronimos Monastery, the Tower of Belém and the Coach Museum is well worth it. Best restaurants for evening eating are in Bairro Alto - take the Gloria lift up.
Big warning about Lisbon - if you arrive late at night after the metro has stopped running then you'll be faced with unmetered cabs at the airport who will try to charge you the outrageous flat fare for a ride into town. This flat fare is, in theory, for journeys out of town but they really have you over a barrel and you could well end up paying €30+ for a ride that is less than €10 on the meter during the day. Any arguments - go into the hotel and ask them to help. Keep your bag in the passenger compartment.
Porto, even more so than Lisbon, is constructed entirely on incredibly steep hills. It is a compact city but expect to lose several kilos if you walk around a lot. I'd strongly recommend one of the hop on/hop off open-top bus companies as a way to get around and do some sightseeing; you can get a ticket valid for two days. If you want a trip outside town, you can book a day coach trip up into the Douro valley to Pinhão in vineyard country including a visit to a winery where port is made. Best places for restaurants in Porto are all around the Cais do Ribeiro at the riverside.
To correct Costa slightly, because I'm anal like that, the port lodges where the stuff is aged in barrels are actually in Vila Nova de Gaia, across the river from Porto.