Nothing is different except the software installed when it ships,paint color, and the addition of a 4th DMX out port.
Hardware wise, these are the only things that changed. It has all the same guts.
NOTE: EMPH-EXPN face panels (Black Emphasis face panels) will revert to Expression 400's when loaded with 3.1 Expression code.
NOTE: EMPH-EXPS facepanels (Single Scene Emphasis Express facepanels) will revert to Express 125's when loaded with 3.1 Express code.