Porto vs. Lisbon

I have not been to Porto so I can't comment about your length of stay there.

We spent 5 nights (4 full days) in Lisbon, and one of those days was a day trip to Sintra. I actually wish we had an extra day in Lisbon to see a few more sites and exploring the city. In addition to seeing important sites (castles, museums, churches, etc.) we love exploring neighborhoods and relaxing at cafes. So I would not decrease your time in Lisbon. I think 4 nights is the minimum number of nights, especially since you are going to Cascais, too.

And you are wise to spend 2 nights in Sintra. I wish we had the time to do that.

This is what we did for sightseeing in Lisbon:
Calouste Gulbenkian Museum - awesome!
Miradouro de Alcantara - one of many scenic viewpoints
Sao Roque Church - Jesuit church
Followed Rick Steves walking tour of Bairro Alto
Castelo De S. Jorge - interesting, great views, and love the peacocks!
The Alfama - we loved wandering around the Alfama neighborhood
Feira Da Ladra Flea market - operates on Tuesdays and Saturdays
Mosteiro De Sao Vicente De Fora - gorgeous blue and white tiles; go to the rooftop terrace for amazing views
Mosteiro dos Jeronimos in Belem - beautiful cloisters; I recommend buying tickets in advance online
Padrao dos Descobrimentos in Belem - took elevator to the top for gorgeous views; no lines
Museu Nacional dos Coches (Coach Museum) in Belem - collection of elegant coaches

Our favorite restaurant in Lisbon is Cervejaria Ramiro recommended by Anthony Bourdain. Fresh seafood!
Also recommend Bonjardim Restaurante for roasted chicken and piri piri sauce.

I think that your time allocation is fine for a first trip. Sintra is easily doable as a daytrip from Lisbon so if you are looking where to shed a night, I would start there and add another night to Porto.

As far as tastings, you will love spending an afternoon doing to Port Warehouses. We did Taylor’s and had lunch at their attached restaurant (Barao Fladgate). Fabulous. The next afternoon we did a reserve tasting at Kopke. Worth every penny and they can ship your purchases home for a reasonable price.

For Lisbon, consider a day tour with Pedro from Lisbon and beyond. We actually did two tours with him and it was wonderful. He can drive you on a day tour of Pena Place and Sintra. We loved our tour.

Lisbon restaurants: Taberna Barrio do Avillezand Prado.

I did a 3-week trip last summer to Portugal and spent 3 nights in Porto and 7 in Lisbon with just 1 overnight in Sintra. I arrived late in the day in Porto so I didn't have that much more time that you will with two full days and I felt it was just right. I think with the time you have you should keep things the way they are. I wish I had two nights in Sintra instead of one, so two sounds like a good idea. I think you could use an extra day in Lisbon, but with nothing to cut, you'll just have to prioritize what you want to see. I would not cut any days from Lisbon to give to Porto. Lisbon has so much more to see/do than Porto IMO and if you only had three days there, I think you would feel like you wished you had more.

Thank you, all, extremely helpful. Will live as is.