Microsoft's take: http://office.microsoft.com/en-us/assistance/HA101205221033.aspx
My take: SharePoint Designer and Expression Web Designer - in my opinion - are not *new* tools - they are a simple evolutionary branch from FrontPage with new function. The split was to create separate products for two separate groups of users - those who use SharePoint and those who do not. There is no way I can use SharePoint on this site - as much as I'd like to, so so that means I fit into the group that would use Expression Web Designer.
IMHO, Microsoft has now blown up the FrontPage name with their clumsy explanation of this split. FrontPage is a *highly* respected and well known brand name. #1 in the business, used by tons of individuals. So far, MS did not handle the communication of this split well - and they are taking a big leap out onto a limb in my humble opinion (as a user of FrontPage back to pre-MS days).
So, as I move to Expression in the next few weeks for this site, everything I've said in the past about FrontPage still applies 100% - because Expression Web Designer is nothing more than the next release of FrontPage.
The graphic to the right in this post is Microsoft's. It's used for Expression Web Designer. Interesting to note that these people are doing work - alledgedly with Expression Web Designer - on a TabletPC. Very cool!
Reference: http://www.microsoft.com/products/expression/en/web_designer/default.mspx