Do you use your phones for GPS at home? If so, why wouldn't you use them in Portugal? It will work the same way, except the units will be kilometers not miles, etc.
The only concern with a smart phone in Europe is the mobile data you might use. Many GPS-type apps have "offline" modes that let you download the maps ahead of time. Google Maps "offline" does for example. Then you can download the map while on WiFi and keep the phone in airplane mode while you drive if you want. if you use Google Maps, try it out "offline" at home to see how it works. It's a little different from the "regular" Google Maps.
But if you will have mobile data in Portugal on your phones, no worries - it will work there like it works here. Do bring a mobile charger ("cigarette lighter" power port) like you would use in a car at home. GPS can eat the battery quickly on a smart phone. Some newer cars have USB charging ports but I wouldn't count on it - would bring your own car charger/USB charging adapter for a cigarette lighter port instead. Their cars use the same cigarette lighter port that is used in US cars.