I think that makes the difference even less in practice. Even a 16x20 print is only a 2x magnification. If both were carefully optimized I'd be surprised (but not really shocked) if most people could tell the difference in such a print.
I have to disagree. A properly made drum scan will easily out shine one made on the Epson 4990 (or V700/V750). The differences are readily apparent in a print, even at sizes less than 2x, at least that has been my experience comparing scans made with a Howtek and 4990. Optimizing a scan from a 4990 just isn't the equivalent to a well done drum scan, even at identical SPI.