As far as getting around, the train will take less time than driving to Coimbra or Porto and will cost about the same for 2 people, once you take into account prices for gas (1.7€/L for gasoline; converts to ~US$8/gallon; diesel is closer to 1.5-1.6€/L), tolls (about 20€ each way between Lisbon and Porto) and the price of the rental.

You do miss out on being able to see small towns along the way.