I'd just add to this, a visit to Birkenau is well worth a trip as well, sometimes it is included as a 'package' of the two but is often missed out which is a crying shame. The size of that place was the most harrowing thing about it to be honest, for as far as you can see the chimney stacks remain from the barrack style accomodation.
We had a middle aged Jewish woman as a tour guide and she was absolutely fantastic, the thing I didn't like was the vast amounts of Chinese tourists climbing on everything they could with stupid grins to take the perfect selfie!