Mercure Gdynia is the joker in the deck - prices are usually modest compared to Gdańsk offerings, while it offers unobstructed sea views and chances of an upgrade in low season are high. Breakfast room is exemplary and so is the spread (for a Mercure). Walking distance from train station, or an Uber ride for the minimal fare (10 PLN) as the distance is so short. Gdynia train station has a direct train connection with the Gdańsk Airport, unlike Gdańsk (you have to change in Wrzeszcz).

Bonus - one of the few Mercures with a full-size (25m) swimming pool!

Ibis Gdańsk Stare Miasto is decent and walking distance from both train station (mind the note above) and the Gdańsk Old Town, but it is an Ibis and IMHO Gdańsk city centre is less appealing that Gdynia to stay at and walk around at night. The affluent Danzigers tend to spend their time and money more on the hills, so wander about and visit e.g. the much-awarded ELIKSIR in Wrzeszcz for evening drinks.

The only upscale option is the Sofitel Grand in Sopot - should be a Sofitel Legend given its history and monumental status, the location is perfect (right on the beach) and is every bit up to Sofitel standards, but not spectacular. You may be better served spending the extra money on some cool dining and excursions. Do wander out of the TriCity as there is not enough to do there at that time of the year. You can e.g. take a day trip to a well-preserved medieval old town of Toruń, where Copernicus lived, or even to Warsaw (3h each way by train). There are also planes to Kraków, available at reasonable prices if you book in advance - again, Mercure Gdynia is best located vs the airport of all the Accor properties in the TriCity.