That depends on how much they need the taxi. I am pretty sure there are no car rentals in Pinhão. They would probably have to rent in Porto, which means they would need to rent for the entire duration of their stay. Taxis they would only hire for the days and missions they need. Whether car is cheaper than taxis depends on much taxi use they intend to make, and the season (summer car rentals are a lot more expensive than winter rentals, but taxis are about the same price year round). It's very hard to tell in advance which will be cheaper.