Just took a train from Warsaw to Cracow this morning. I disagree with some posts here. Warsaw has much to offer besides the Old Town and the Castle.
Old communist era relics are still around. Palac Kultury, a gift from Soviet Union to the Polish people is worth a visit.
Lancut and Lazienki Palace and park are just wonderful places to visit.
The Jewish uprising and the Warsaw uprising museums are fascinating.
Many outstanding restaurants, great nightlife, great shopping, etc...
I stayed at the very centrally located Residence Diana. Just about every major tourist attraction is walkable from the hotel. A wonderful and modern apartments , but also a quiet oasis in the midst of action. Highly recommend.
www.residencediana.pl (Great studio apt number 103)
After arriving in Cracow I've checked into doing the Auschwitz and Salt Mines tours. It's possible to do both in one (albeit) loooong day. The same company offers morning Auschwitz tour and then after a short break an afternoon Salt Mines tour. Both tours are $48 each.
Wawel Castle and Old Town can be done in 1 day as well.
I'm staying at the Kazimierz Deluxe Apartments. Kazimierz is the old Jewish neighborhood, just outside of the old historic walled in Cracow. It's quickly becoming the Soho of Cracow with many galeries, jazz bars, great little restaurants. It's only about 5-10 minute walk to the Wawel Castle from the apt. (
http://www.krakowlife.pl/362,Kazimie...Apartament.php) and it's fabulous. Just recently finished with full kitchens, a living room, separate but connected bedroom area, huge modern bathroom, and in my case a loft with a sitting area. Again, great location and at 70E per night, unbeatable. (my apt number is 9, in case you decide on this place, it's the Silver apt).
Warsaw, more modern, more cosmopolitan, Cracow, historic, fun.
You will have to make the call.
Have a great trip!