Is it worth staying in Sintra for 2 nights instead of just staying in Lisbon

It's up to you, but Sintra is only 39 minutes away by very frequent trains from Lisboa Rossio station and changing hotels is a time consuming effort (check in/unpack/repack/check out/transfer/check in/unpack, then repeat again). I never change hotel accommodations if two locations are so close to each other for that reason. I'd choose one location and do day trips from there, and in your case Lisbon makes the most sense.

It depends on what you are wanting to do in Sintra. Sintra can be hard to manage. It is also a very atmospheric town with a great restaurant (In Commun) For those reasons, we chose to spend 2 nights in Sintra. We don't find that changing accommodations takes much time at all. If you want to see multiple palaces and/or actually visit the inside of Pena, it worked well for us to get to Pena at about 9am. Depending on how early you are willing to get up, that might be difficult to accomplish if you commute from Lisbon.

If you are saying that you want to spend two days in Sintra, then you only have two days for Lisbon. Also, I think the idea of staying over in Sintra makes so much more sense than commuting to Sintra twice.

We don't use Airbnb for short stays, and much at all, for that matter. Prices always look good until you add in fees and cleaning fees. We also like the advantage of having folks on site to answer questions or just to chat a bit.

You could consider 1 night in Sintra. Get out of Lisbon early, go to Sintra, drop luggage at accommodation, visit Regailera and Monserrate, possibly National. The next day get to Pena at 9am, visit, perhaps go to the Moorish castle after, then go back to Lisbon. Ideally, you'd have a place to store your luggage.

Just to give you a feel for what we did, we took the train immediately from the Lisbon airport to Sintra (I believe we took a metro to a train station). We then traveled all over Portugal and finished in Lisbon.

I would stay in Lisbon for the 5 nights. I spent 8 nights in Lisbon earlier this year and went to Sintra twice.

If you are in Lisbon the whole time, you will have flexibility on which days to visit Sintra. You might want to stay in Lisbon if the weather is not ideal.

You might find my trip report helpful:

https://community.ricksteves.com/travel-forum/trip-reports/solo-in-lisbon-portugal-my-trip-report

We stayed in Sintra 2 nights and were glad we did. We could have a somewhat leisurely morning and still get out to the sites by 9:30. We went in late September and it was still crowded, so expect lines for most of the attractions. Sites close between 6 and 7 p.m., so you can go later and miss some of the crowds. If you like plants, most of the grounds are huge (but not formally designed) so they are nice to wander around if the weather is fine.

I can't speak to Sintra nightlife. We generally got back into town around 7 or 7:30, then looked for someplace to eat. By around 9 we were heading back to the guesthouse for the evening.
It sounds like you're leaning towards staying in Lisbon, and that will work for you. You may find you did everything you want to in one day and that's one more day in Lisbon (where there are infinite things to do).

Thank you all for the input. I think we will stay in Lisbon for the whole time.

The most important factor is that this option gives us flexibility. We can decide if we want to go to Sintra for one day or two days. And we can decide when to go, depending on the weather.

Dedicating two nights to Sintra is a good idea and I wish we had. Since it's such a short train ride it probably makes sense in your case to remain in Lisbon. We had bad weather near the end of our day in Sintra which made footing treacherous at Pena Palace and the Moorish Castle (my favourite). By staying in Lisbon you will have the flexibility of choosing dates around the weather.

I'm so glad I found this post. We have booked 4 days in Lisbon and 2 days in Sintra. We are now rethinking our plan with possibly the whole 6 days in Lisbon with 2 day trips to Sintra. We plan on going in September and I know Sintra shut down for a few days this past September due to the threat of the Porto bushfire. Also this gives the option of still having 2 slowed down days without the baggage worry..as well as weather issues

HULA, Changing your plans to all nights in Lisbon would give you more flexibility. And like you mentioned, it is nice to move around without luggage. My recent trip to Lisbon was the first time I stayed in one location and one hotel for the entire trip. I really enjoyed not having to haul my suitcase from place to place. And there’s plenty to keep you busy in Lisbon.