There were some updates to the item descriptions after I asked this question - on the Amazon UK site as well. The more expensive CD (B000O78K5G) has a slightly different track listing and some extra songs. The other CD has a better description on the UK site - http://www.amazon.co.uk/Nb-Natasha-Bedingfield/dp/B000NJLPTS/ref=sr_11_1/202-0708237-0118267?ie=UTF8&qid=1176816213&sr=11-1
The Amazon.com/Amazon.co.uk descriptions have been updated yet again. There is no mention of any song called Stepping Stone. Now it looks like the only difference, aside from $8, is that the more expensive CD has a data track and "Unwritten" from her 1st album. I have the $21.98 CD. The data track grants access to "Exclusive Video Content": 3 wallpaper designs (that are available for free on the website) and an 8 1/2 minute documentary on the making of the "I Wanna Have Your Babies" video. To enter the Exclusive Content area, you have to sign up for the mailing list.