Well I've just finished the prerender portion and have compared the two methods.

Loose Bitmapdata's
Zero-usage 9547776
Memusage 16097280
delta 6549504


Spritesheets
Zero Usage 9461760 (the test main class defines less instance variables/functions)
Memusage 9764864
delta 303104

Spritesheets vs Loose Bitmapdata's
4.6% vs 100%

Now I understand the hype around spritesheets! I also expect a speed increase in the rendering method, as I've set up the spritesheets to be directly accesible.

Oh, and the dispose method didn't quite work. I think it completely trashes the bitmapdata, as I got invaled bitmapdata errors. Using fillrect() instead did the trick.