Personally, I was put off the coach tours after reading reviews on the Tripadvisor and Virtual tourist websites.
The consensus seemed to be that they were hastily shown round without the opportunity to spend time, or 'quietly' reflect on the things they saw.
Entrance to the two sites is free and the guide book is only a few pence, far better to go at your own pace.
You can engage a guide if you want to, but everything is in English so it's not really necessary.
http://www.auschwitz.org.pl/html/eng/start/ind ex.php
It's a very personal and harrowing experience for most people (regardless of faith), there were four in our party and we didn't exchange more than a couple of words for several hours. For me, It would have almost been disrespectful not to have witnessed the sights at my own pace, and I couldn't have done that as part of a large (and by definition, a fairly noisy) group.