Another difference I spotted is about onEnterFrame calls.
When removeMovieClip is used, no onEnterFrame is invoked.
When unloadMovie is used, the first onEnterFrame is used.

See this:

createEmptyMovieClip('a', 1);
a.onEnterFrame = function() {
        trace(this+' enter frame');
};
a.createEmptyMovieClip('b', 2);
a.b.onEnterFrame = function() {
        trace(this+' enter frame');
};
a.b.onUnload = function() {
        trace(this+' unload');
};

a.removeMovieClip();
a.unloadMovie();

Other difference: onUnload doesn't really destroy the DisplayObject, but only its contents, compare the following:

        createEmptyMovieClip('a', 1);
        a.onUnload = function() { trace('unloaded'); };
        _root.onUnload = function() { trace('unloaded _root'); };

        a.lineStyle(10, 0);
        a.moveTo(100, 100);
        a.lineTo(200, 200);

        trace(a.getBounds().xMin);

        a.removeMovieClip();
        a.unloadMovie();

        fun = function() {
                trace("fun, a is "+typeof(a));
                a.loadMovie('green.jpg');
        };

        setTimeout(fun, 1);
        trace(a.getBounds().xMin);

The removeMovieClip vs. unloadMovie.
Make sure green.jpg is in the same dir the .as above is.
You'll notice that after a.unloadMovie you can still load into it
while 'a' is undefined after removeMovieClip.

It seems that the bug exposed by my patch is indeed also a bug for the .removeMovieClip case. That is getBounds still return the full bounds of an unloaded/removed clip before it really gets destroyed.
A simple testcase:

 createEmptyMovieClip('a', 1);
 a.onUnload = function() { trace('unloaded'); };
 
 a.lineStyle(10, 0);
 a.moveTo(100, 100);
 a.lineTo(200, 200);
 
 trace(a.getBounds().xMin);
 
 a.removeMovieClip();
 //a.unloadMovie();
 
 trace(a.getBounds().xMin);
 
You'll notice NO difference in the removeMovieClip and unloadMovie. If anyone feels like trying harder to make them different it will help.

Attached is a wonderful game which suffer from a blinking
wording not going away. UNIMPLEMENTED unloadMovieclip sounded
like the culprit so I tried implementing that as MovieClip.unload() and indeed fixed that game.

Well, this bug is a reminder as I don't have time to do the
fix properly, but for sure this version does fix that game