In Part 1 of this post I tried to explain SharePoint editions in more detail. Last week I found very interesting shema that describes version differences on a single image. You can download the full presentation here. The complete breakdown of the features of Microsoft Office SharePoint Server 2007 is available here.
When choosing the correct SharePoint edition you must also consider licensing. Some SharePoint features like Excel Services and e-Forms are great but these are only available in the most expensive Enterprise version. Please note: enterprise search is also available as a separate product Search Server 2008, and e-Forms are available as Forms Server 2007. So before advising your customer which edition is right for them, consult licensing documents carefully.
If you are trying to find correct licensing for extranet and internet scenarios take a look at the chart below. I got it from guys at Microsoft Croatia and find it to be a valuable asset in deciding which edition to purchase.