The MBA SuperDrive isn't a current product, which may be why you're finding it cheaper somewhere. The USB Superdrive is the current model.
I'm guessing there's no real difference; they just called it the MBA Superdrive originally since the MBA was the first Mac to lose built-in optical. Now that most of them don't have it, "USB Superdrive" is a more useful name.
I don't see cable length specs anywhere but it's possible that the cable is shorter on the MBA version; they may have lengthened it to accommodate the iMacs.