I voted MTW, as that's the only game I have modded in a very basic way (changing names, and giving units to this or that faction etc).

So if even I can "mod" MTW, it really can't be that hard.

But the thing is that I haven't even tried to do this with RTW. Should probably just be a matter of spending some more time studying the file structure, but as I don't have that much time I guess that was the end of the mini-modding story for me...