On my system, both binaries point to
max@linux-vwzy:/> which mail
max@linux-vwzy:/> ls -la /usr/bin/mail
lrwxrwxrwx 1 root root 5 Mar 2 2011 /usr/bin/mail -> mailx
max@linux-vwzy:/> which Mail
max@linux-vwzy:/> ls -la /usr/bin/Mail
lrwxrwxrwx 1 root root 5 Mar 2 2011 /usr/bin/Mail -> mailx
Do the same as me to see where
Mail are and what they are linked too. That will tell you the details of your system. You're saying you get an error, which leads me to believe that they are NOT pointing to the same application.
According to this article,
Mail are extended versions of the
mail command. They have more options, which is why you might be getting errors.