Lisbon/Porto

What is the appeal of Comibra to you? What are you expecting?

It is a neat town with character, but it's very small. If you love universities, you might enjoy the university tour. If you're response to that is, "What university?" then maybe you aren't going to get much out of the town. On the upside, it's very untouristy, compared to Porto and especially to Lisbon.

A day stop would probably suffice - but that would be a lot easier with a car. There is no official bag storage at the Coimba train stations, and to get into Coimbra by train, you have to change at Coimbra B (potentially both ways). There are places to leave your bags, though (forget which recently solution someone suggested - the TI?), just no lockers or left luggage at the stations.

Lisbon 4N = 3 days (1 in Sintra)
Coimbra 0-1N
Porto 2-3N = 1-2 days

It is possible to stop in Coimbra if you find luggage storage (or stay overnight). Take a taxi from/to Coimbra-B to/from the historic center and do the RS walking tour. Don't waste time waiting for the connection to Coimbra. Taxis are cheap. If you stay overnight, take an early train to Porto and drop your luggage - bear in mind that if you're not staying in a hotel in Porto, you may not be able to do that.

Take the train, driving will take longer (including the time to rent and return the car) and you'd need a large car for 5 adults and luggage. The AP trains are comfortable and the line from Lisbon to Porto goes through Coimbra.

Whether you

Thank you for the comments, very helpful advise!
We might hire a private transfer from Lisbon to Porto that includes a 2 hr stop at Comibra.
If we decide to do just a stop and not overnight, should add that extra night in Lisbon instead of Porto?
A lot of posts here are suggesting that 2 full days are enough.
Thank you again.
Grace

Coimbra is nice, but I would devote your time in the north to the Douro River valley.

We did a 7 night river cruise and it was fantastic.
Take the train, it runs on the north side of the river. Go in to Regua and beyond if possible. Lots to see and do.

Coimbra has a great ancient Roman museum which incorporates ruins in its lowest level. We had limited time, but could have spent many more hours there. If you are interested in both the university and the museum you can fill a full day there.

We narrowed down to 5 nights in Lisbon and 3 nights in Porto.
Any advice for Alfama vs Chiado for our lodging?
We are all healthy and fit so hills are no problem. Just can't decide between 2 wonderful place we found in each town.

Thank you.