With a larger cabinet and two woofers as opposed to one, this will likely improve the soundstage with the SCS.
I imagine the price difference would be worth upgrading the front LCRs. From all the pro reviews on the SBS/SCS speakers, they seem to perform quite well for movie/HT usage, though if music is a larger priority than you've noted (70/30 movies <-> music), there may be others you want to look into... -TD