For future reference as I usually forget myself, the difference between the two is quite simple as this SuperUser post puts it:
useraddis native binary compiled with the system. But,
adduseris a perl script which uses
useraddbinary in back-end.
adduseris more user friendly and interactive than its back-end
useradd. There’s no difference in features provided.
Found here: http://superuser.com/a/547973.
You’ll find on some systems that the
adduser will give you prompts for adding extra information.