Your ship must be docking way out from the city part of Stockholm. When we caught our small ship cruise in Stockholm, it was about a five-minute cab ride across the bridge from the main area of Stockholm.
I think it will be a matter if you want to be mostly inside and like historic ships (and the saga around this one). It is interesting, but relatively dark inside. Or, if you want to be outside a bit more.
I would suggest looking at on-line photos of your top two choices and deciding what is BEST for YOU.
Since you have referenced your age a few times, I am guessing it may be a matter of your particular physical ability. We were with an 80-year old and a 70-year old when we visited Stockholm, and we walked all over the place during two days.
You might also consider visiting the sites of the Nobel Peace Prize ceremonies and City Hall, if that is a shore excursion choice. We took a guided tour of City Hall, which was very interesting, but I do recall a good number of stairs.........and we also climbed to the top of the tower (obviously a lot of stairs).
The chapel by the Palace (can't recall its name now) was very pretty and interesting.....site of lots of Royal History.
Enjoy your trip.....but do something that feels right to YOU.
If you post here what is specifically included (the description) in the shore excursion the CSR recommended, we can react to that.
It can be very chilly in Stockholm, especially on the water, so be ready with layers, just in case.