I've never been to St. Petersburg, unfortunately.
But I've seen many pictures, and I've heard it's a great city.
Been there several time over the last six months. I find Krakow being very drab, depressed and trying to get over the Soviet Occupation. There is a split between the old and young. Everything is now being privatized and the old are loosing the governemtent stipend. The young are excited for their newfound freedom.
Went to a park in the middle of Krakow one evening and listened to a group of Polish kids sing songs (not organized) it was one of the most moving experiences, the songs were of hope and freedom. Just below the Cathedral in Krakow.