This is what I've found:
Sopot is mainly New Year's Eve on the pier. Gathered people can count on a great, as every year, fireworks, great shimmering above the surface of the Baltic Sea as well as a wide range of pubs, restaurants and clubs preparing an excellent fun. The decision on how to spend New Year's Eve in the Tri-City is an individual choice, one thing is sure - the opportunity to be a one time lived with great fanfare will be enough.
Look also at: https://polishforums.com/travel/best-places-visit-sopot-december-72928/
This year's New Year's Eve on the Market Square in Krakow promises to be a truly spectacular! The event will once again supported and transmitted by TVN station.
New Year's Eve 2014/2015 promises to be as spectacular as in the last year - it will be a big multimedia scene and known hits played non-stop! Agnieszka Chylińska will perform there, Maciej Maleńczuk, Kayah, Zakopower, IRA, Wilki, Feel, Grzegorz Hyży, Red Lips and Future Folk. Agustin Egurrola will take care of the choreography and also dancers from the VOLT group.
There will also be familiar faces from TVN. New Year's Eve concert will be hosted by popular presenters of the station like Agnieszka Szulim, Patrycja Kazadi, Marcin Prokop and Olivier Janiak.
Sopot has some very good pubs and night clubs, and is much smaller place than Krakow, but also crowdy.
Kraków - beautiful city, bigger event at New Years eve, so I would probably choose Krakow.