If you intend using public transport being based on central Lisbon is advisable as transport links radiate out from the centre of Lisbon.
If you have a car, then being based in Estoril or Cascais is not really a problem, except if you want to travel at peak times to places such as Estremoz when the Lisbon traffic can be heavy.
A good option might be to stay somewhere like the Best Western Riviera in Carcavelos. This is half way between Lisbon and Cascais and there is good access to the Lisbon motorway, the linha de Cascais Rail Service and the Marginal Coastal road.
