For future reference as I usually forget myself, the difference between the two is quite simple as this SuperUser post puts it:

useradd is native binary compiled with the system. But, adduser is a perl script which uses useradd binary in back-end.

adduser is 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.