Just use the simple version at all sizes. Why does an icon need fine detail? Icons are abstract representations, like letters. Do you insist on adding ornamentation to your letters at large font sizes?
That's a great analogy. For scaled outline fonts to remain legible at low resolutions, they have a sophisticated hinting system (it even has a little virtual machine!).
I'm sure if someone developed a hinting VM framework for vector icons (that wasn't patent-encumbered like half of the font ones), it would do a lot to help convince people that special low-resolution bitmaps weren't necessary for tiny icons to look good.
Actually, fine font serifs are visible only in large sizes. Curves also can be more more finely rendered and gain a lot from bigger size/better resolution.
People do recommend switching between serif and sans serif typefaces, depending on the size of the text. Admittedly, it often goes the other way—-sans serif at bigger sizes—-but the idea is the same: some things look better at a particular size.
