I realize, Misschris, that being a national holiday might cause some of the sites and shops to be closed, being Poland’s Independence Day. That is certainly a valid consideration when deciding to visit Krakow or Helsinki. But there is another consideration as well, a positive possibility.
A few years ago, I was on a trip in Spain (not an RS trip) and we were told that on a particular day there would be a national holiday, and that many shops, banks, restaurants etc., would probably be closed. Indeed, we found that to be the case when we arrived. (As an aside, many restaurants and shops, especially those catering to tourists, were open, though the banks and post offices were closed. ATM were plentiful and functioning, however). The positive note is that we also encountered various holiday processions, both secular and religious, and other types of festivities, things out of the ordinary. Things we would have not encountered and enjoyed had we been there on just an “ordinary” day. Think of the Fourth of July 🇺🇸. Yes, there are many things that are closed for the holiday, but on the other hand, there are the parades, the fireworks, the other celebrations. So what I am suggesting, tho some things might be closed, the local celebrations might give you a unique taste of the city, country. A trade-off. Only you can decide.
Also, in my humble opinion: Krakow v/ Helsinki? I would definitely opt for Krakow. Not saying anything negative of Helsinki, a lovely city. But no match, in my estimation, to Krakow.