You have not stated when you are travelling. I live in Lagos and would always recommend a visit here.
However, you are in Lisbon for only a week. Cascais is only 25km from Lisbon and although the sea front is perhaps not as natural as Lagos, it is very pleasant.
To be direct, I think that if you are visiting Lisbon in the run up to winter, there are other places better worth visiting than Lagos at this time of year given the travel time.
You will need at least 3 days in and around the Lisbon/Sintra area. I would recommend you consider alternative days trips (by train) to Porto and/or Coimbra and by Bus to Evora. There are historic towns such as Obidos, Alcobaça and Batalha which can be easily visited in a day from Lisbon on organised tours.
