open-watcom-c-dos-1.9.7z    DOS only (host, target) with all DOS-related files
open-watcom-c-dos-1.9.exe   DOS installer (.ZIP sfx) w/ all hosts and targets

In other words, if all you want is a full DOS install (C/C++, help files,
extenders, utils, stub sources), you only need the .7z file. The .EXE is 
the "complete" / official install of everything (including non-DOS related
stuff, e.g. Windows, OS/2, etc).