I would highly recommend you staying at Mario's. His place is lovely and near the city center. Mario is always reachable by texting him if you have any question. His map of the places you have to visit in porto is really useable. The map is also developed as an app to download and you can use it without internet access in the city. The breakfast vouchers you get are also a nice gift. You get a hot drink, a cold one and something to eat :D.

If I visit Porto again, I hope I can stay there a second time.

Cheers,
Thomas