In my opinion there is little comparison between the S and STX. The STX is over a ounce lighter that than the S and packs more features, it has dual braking, the infini II spool, more bearings in key locations, a smoother retrieve and a better handle.
The spool and weight of the reel make it more than worth the $70 dollars for me, throw in the extra bearings and dual braking, and as I said before, not much comparison in my opinion.