I though the English version of FoV and DKS were both set for an October release? The German FoV will be released mid August.
FoV does include engine updates, bug fixes, etc for D2:ED. DKS changes actual content of the D2:ED section of the game (so far the flying fortresses seem to be the biggest difference announced). DKS may include more updates or texture changes, etc, than are included with FoV, but if so it has not been mention to what extent.
