I loved Mario's place. It's super convenient to walk around Porto and you can easily walk into Gaia (that part is all downhill--you have to go uphill to get back). Plus, there are nearby transit options. There are restaurants on the same street, so it's in an awesome location, and even more if you walk to the main streets.
Mario's place is spacious and is super neat and clean. One of my favorite parts was the bath towels--incredibly thick and soft. There was plenty of hot water. It's very quiet and I could only hear minimal noises from the hallway. The locks were easy to use (I have a hard time with locks so feel that is worth noting).
Do note that there are two sets of stairs, but I could get my insane amount of luggage up them. No issues with Ubers getting to this location. though if you don't have a ton of luggage and you don't mind an uphill walk, you can walk from the Sao Bento station or it's a short ride from Campanha.
Mario and his friend were super friendly and responsive, which is truly appreciated. I would absolutely stay here again. Both the apartment and the host are great and it's the perfect place to stay in Porto.