I don't have the Express. From what I understand, it is the same as the standard Big Shot except it is motorized. Hopefully someone else will jump in with their thoughts on the motorized version.
As far as die compatibility goes, it can take most of the Sizzix dies. It will take the Bigz no problem. It will take the Bigz XL if you've got the longer (XL) cutting plates.
It is NOT compatible with the PRO dies, which are big and only compatible with the pro machine. A good place to look up compatibility is the Sizzix site:
Sizzix.com - Die-Cutting and Embossing Tools for Today?s Crafter