Sintra or Cascais ??

We were just in Portugal in April/May of this year. We spent almost a month.

Cascais is lovely. Unfortunately, the day we went it was extremely crowded and hot.

Are you interested in seeing the palaces in Sintra? We really enjoyed the ones we went to, it was a highlight of our almost month long trip to Portugal. If you are not interested in the palaces, then I would see no reason to go to Sintra, and I'd pick Cascais.

Mid October is lower tourist season and a great time to be in Cascais. It is definitely less crowded than Sintra. I’d encourage you to look at pictures and read a bit about both towns. You would be much closer to the palaces in Sintra. Cascais doesn’t have the blockbuster sites but is a lovely town with lots of good restaurants and nice small beaches.

We will be in Cascais in mid-October during our fourth visit to Portugal. I have been to both Sintra and Cascais in the past and believe that Cascais in October (post high season) would be more interesting.

If you are in Sintra for a night, I don't know if the castles are even open late. Other than the castles, there is not much there there in my opinion.
Dennis

Two completely different atmospheres so it depends on what you like, or want, to do that night.