The difference is mainly in the tone itself. The output of the pick up matters very little because your amp can make up for any difference in output. The '57 classic's have a different sound balance. ( lows, mids, and highs.) you'll notice it more if you play clean like jazz riffs or fingerpicking.
In my opion the 490's have a more Rock oriented tone as to where the '57 Classic's are a more all around tone. Sutable for Jazz, Country, Rock, Blues, etc.. That may be why Gibson puts them in more of the hollowbodies over the other pickups.