Porto seems so completely different when compared to Lisbon, that visiting it is almost like going abroad. This is difficult to explain, but all of the Lisboetas I talked to, including those born in Porto, share the same feeling. Porto is much slower. More quiet. A little bit less relaxed. You do not hear that much music on the streets.
If you have ever wondered what is the difference, if any, between the “southern” and the “northern” life style, try visiting Porto and Lisbon on the same, sunny day. You will experience a cultural shock.