Join Date: Feb 2009
Bikes: 2008 Mercier Corvus Steel, 2007 Trek 4300, ~1980 Takara Deluxe 12, 1985 Trek 620 (modernized)
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JiveTurkey is right. The derailleurs are a slightly different shape and width. Theoretically, if you lined up both derailleurs in an identical way, you'd get more rub in cross-chained gears. That said, the difference is so small that even a fraction of a mm in cable adjustment will render the difference negligible.
I agree that when the price is low enough, I'd get the correct version just to give myself piece-of-mind, because the differences are not just in the width. The newer iterations are almost always machined and shaped in a more advanced way.