We just published a whitepaper comparing Express and Compact Edition.  The reality is the paper tends to focus less on a direct comparison of the products, but rather an evaluation of what’s needed in a local data store.  Express edition is our Data Service entry point from single user to multi-user scenarios.  With the release of Compact Edition, we are positioning Compact Edition as the default local database and will focus local data feature in Compact Edition, and low end Data Service scenarios around Express.  This point is that one size doesn’t fit all, but two definitely conquer a tremendous amount of territory.
http://download.microsoft.com/download/A/4/7/A47B7B0E-976D-4F49-B15D-F02ADE638EBE/Compact_Express_Comparison.doc

Steve