Cascais and Estoril are essentially suburbs of Lisbon (yes they are 30 km away, but most of the "important" Portuguese live there. YEs they are nice for being so near a European city, but if beautiful beaches, diving etc are what you are into, then these may not be the places for you. The Algarve is more resorty.

Do not get me wrong I have been there several times but they do not seem to be the type of place you want.