I found Sintra to be magnificent but unfortunately due to my arthritis, walking was limited.
It was a sometimes pretty train ride from Rossio Station to Sintra. Once there, many persons are wanting to sell you tours or transport to the top. There is also city buses that transport.
I chose to walk some on the path going up , I could not walk much but from what I did walk, it was really awesome. Nature was beautiful. There was architecture and statues and exhibits.
There were museums. One was below the path right on the ground and of course, I could see the palaces and chalets and castles and homes of the wealthy as I walked.
I was not able to get close by foot. Some persons chose to walk uphill but as also said, there is also many different transports to get to the top
There are many restaurants to eat at and shops to shop at.
If possible, Sintra should be included. In your travel plans.
Belam was a nice bus ride from Rossio Plaza. but the bus does get packed
We passed through some very pretty neighborhoods that I would not have seen otherwise
Belem was interesting and while not as beautiful as Sintra was still very pretty with statues, architecture, pools and sites. Lots to see .
The lines were so long at the monastery that I chose not to try to go in. There are other places to enter with shorter or no lines. There are several museums with the bigger one having to do with navigation. Very interesting museum gift shop.
I chose to simply walk around.
There is a nice cafeteria which I ate at. Had salad, soup and dessert. There are also plates of the day which looked tasty for about 10 euros.
Belem was very pretty and interesting
If you can go to both places, that would be the best option but if you can only go to One, I would say Sintra.