If this is your first trip to Lisbon and you do not intend to make any day trips to places like Obidos, Alcobaça or Batalha, strongly recommend you take the bus or train.
1) It's cheaper! Fuel is expensive and there are tolls on the A2 motorway running from Paderne all the way up to Lisbon.
2) Lisbon Council have put in place very strict traffic calming measures in both the Baixa and the Alfama/Castelo area. If you are staying near the Castelo, I assume you are staying at a Hotel like the Hotel Olissippo Castelo. Even for someone who knows Lisbon getting in and out of there is a challenge. You will probably have to depend on expensive hotel parking or expensive public parking.
3) Lisbon has excellent public transport, be it the Metro, Tram, Bus and suburban lines to Sintra, Cascais etc.
4) If you are staying in the area methinks you are staying in, provided you are prepared to walk, it is only 7 to 10 minutes down to Rossio/Restauradores where you will find Buses, Metro and the Train to Sintra.
Chiado is about 15 minutes by foot allowing for the descent and climb.
The train is a great way to travel to Lisbon. If you are heading for the Castelo area, get off at Entrecampos Station and grab a taxi.
En route you will see the lovely Alentejo and also will have a great view of the Tejo Estuary and downtown Lisbon from the 25 de Abril Bridge as the train crosses it.
On return, take the train to Faro and jump a cab outside the station straight to the airport.
It will work out a lot cheaper and less stressful.
I have driven in Lisbon for years but I now almost only travel up by train or bus as it is much less stressful that having a car when one is there.
