Krakow or Prague

Prague is certainly the more popular site and does have the wonderful distinction of being a great old city that was never bombed, so all of the original architecture is there without in typical 1960's infill. Many people love it. However, while I found it visually appealing, I also found it culturally vacant. No sign of Czech life going on anywhere a tourist would walk. Krakow was a wonderful surprise. Wonderful atmosphere in a very managable size with locals who still live there and seem very happy to see you. As a bonus, the costs are much lower than any city to the west, including Prague. If you are interested, Auschwitz is just an hour away. Krakow can be seen in 2 days (not counting Auschwitz). With 5 days, you could get a reasonable taste of both.

They are both beautiful cities, but I second Randy's vote for Krakow. Okay, I'm biased because I have friends there and many, many happy memories, but I think Krakow truly has a lot to offer. Some may say that four days is more than enough time considering the relatively few "must see's", but if you like to explore beyond top castles and museums, there's plenty to occupy your time. As Randy alluded, Krakow still has (or had when I was last there five years ago) a real local feel. There's a major university there so it also has a youthful, energetic vibe: lots of little cafes and pubs with students listening to jazz or discussing politics. And on most weekends, there's some sort of concert or festival in the main square which attracts locals and tourists alike.

I vote to visit them both if you can spare another day or two. I prefer Krakow to Prague only because the later is overun with tourists. You could consider taking the night train between the two cities to save time. You can get a private compartment with a sleeper on the train.

Thanks for all the suggestions.