Mail mode vs Message mode

[BP25]

Hello!

I'm choosing which mail composition method to use betweem mailmode
(sendmail.el) and messagemode (message.el). I wish there were a
comprehensive compare and contrast on the Emacs manual!

I gather the main difference is that one provides MIME-support (whatever
this means). Are they being developped "in parallel"? Do they have
overlapping functionalities which are implemented differently? Why not
just merge them into one single MIME-compliant mode which is compatible
with both: this will spare so much confusion to newbies!

Please let me know! (and please CC: my email address because I'm not
subscribed to this list)