The main difference between the 2006 and the 2008 is that the centre display was improved in 2007 so that it has more pixels. Other than that it's a wash. The body style is the same from 2004-2009 the only differences being minor changes to the tail lights and where the badges go.
There is no practical difference between 42000 km and 56000km. If it's important to you, the newer one will have more years left on the battery warranty, but batteries aren't a problem with the Prius. Failures have been few.
My 2004 is still going strong at 125,000 trouble-free miles ( just over 200,000 km ).
http://www.ctvbc.ctv.ca/servlet/an/l...shColumbiaHome