For Tableau Server - absolutely no difference.
TWBX is an packaged workbook (an archive) that contains TWB and all the necessary files (extracts, images etc.) and is meant for sharing.
For Tableau Desktop there's an extra cost opening TWBX and it has to be unpackaged into temporary location before it workbook can be used (which takes time).
Tableau Server unpackages TWBX workbooks on publishing and stores them split out in it's own special way to behavior is identical between TWB and TWBX files there.