Lisbon to Porto

It's about 2:45 by train. Why would you even consider flying?

Train is fast and easy.

Much easier to take the train. And, very cheap. Round trip is around €25 if you are over 65.

While the trains in Portugal aren’t as nice as those in some other European countries, they are quite comfortable. After living here for five years, I would never consider flying between Lisbon and Porto.

We did a tour with Douro River cruise that started in Lisbon, then by bus to Porto for the cruise. After the cruise we stayed a couple of days in Porto and then took the fast train back to Lisbon for our flight. Train was fast and efficient, while a bit dated.

The only reason to fly would be to catch a connection through Lisbon to a city you couldn't get to from Porto (say, Porto to Lisbon to Salvador, Brasil). Otherwise, the flight will take much more time and be much more complicated than taking the train (or even the bus) between the two.

We flew BOS->LIS, took the RS Portugal tour and then flew Porto->LIS->BOS on the way home. The Porto airport is easily reachable by Metro from the city center. It is a much busier airport than I expected with flights to all over Europe. If I remember correctly we either didn't pay more or we paid minimally more adding the Porta->LIS leg to the return flight and we saved several hours on our total travel time. Most people on our tour took the train back to Lisbon for their return flights to the US. If you can arrive at one city and leave from the other city I would fly otherwise I would take the train.

Unless you are already at the Lisbon airport ( e.g., on an arriving flight) it looks to be easier to do land transit between the two.