When we arrived at Auschwitz (by train from Kracow) it was after 10am which meant that we would have been obliged to join a compulsory 3 hour guided tour. This tour would have included a 20 minute video which we did not want to see and it was only possible to leave the tour at designated points with no opportunity to see anything without a guide.
We decided to take the shuttle bus the 2km to Birkenhau (or Auschwitz 2) and definitely made the right decision. We were able to wander at our own pace around the deserted and enormous site and chose where we lingered and spent time with what we felt was important to us.
We took a bus back to Kracow which went through many villages and gave us a chance to see more of this small part of Poland.