I would definitely recommend staying at Cristina's place.
The location is great, a metro station nearby, you can also reach the airport by metro.
My friend and I really enjoyed staying there and we visited the city, Sintra and Belém. Everything was very easy to reach by metro or train.
Everything we needed was available in her apartment.
Clean bathroom, kitchen, room and even a litte handmade book with information about the city. Very nice.
Thank you Cristina and her family.