After travelling to Portugal now about twenty times i have hire quite a few cars.
At Porto airport you gets picked up and taken to the place where all the hire companies are situated. On two of my trips i found after turning up to pick up my car only to find that i was not getting a car. The reason being was that i only had a personal Debit card. What i needed was a credit card, i only have a company credit card and it's not for personal use. On each of these two occasions i had to walk around all the hire companies until i got a car.
I tried to use my company card but because it was not in the name of the hirer i was refused again.
One company i did find helpful was called Economycarhire now these people use a company called Rentauto to supply the hire car. At the Porto airport you get met by the company reps and taken to the hire company office. At the office unlike the other companies you are expected and the papers are ready for your arrival. Nice comfy chairs to sit in unlike the standing in line at the other companies. The service is by far the best i have met on all my trips to Portugal. At Faro airport on the two flights into there i have made i got the same excellent service.
If i had a credit card i would never have had the problems i have had or i would never have found a company who provides such excellent service. My reason for a Debit card was simple if it gets lost or it's stolen then all i could loose was the amount of money i had on the card.