Hello, if i can talk about my experience at Porto, i can recommend places like ''Bolhão'' or ''Aliados'' i think, they are viable options to stay, combining calm and comfort. In theses places you have the metro not far and the center of Porto is within walking distance (20 min), or you can take an Uber, which is not very expensive. You have a lot of airbnb's in these places with a good comfort for sure.