They sound very much the same other than bass, I am talking S Towers, Non S THX towers and on THX Monitor so, I have no experience with THX S Monitors.
There should be some clues below BUT so far my favorite setup is THX S Towers and Non S THX full sized center for all but large rooms, towers are clearer as they have a midrange more or less (2.5 way vs 2 way). Looking at the towers (S) and the monitors (Full, non S) you are 500 bucks short, to me this is a no Brainer if you can budget it. Of course we are talking my room and my ears ( and REW). If you disagree, you can ship them back on their dime within 60 days, only downside. Unboxing, reboxing and maybe stairs.
If you must be close to a wall, say 2 feet and have a small room room gain on the S's. is easy to deal with, not as easy with the 1723 but can be done with crossovers and a bit of work.
After playing with them for a year that seems to be the rule, small room and/or close to the walls, S, big room, can pull them away from the walls, non S.
In a small room you will get all the bass you want from the S's. The non S tend to overload a room at higher volumes.
How close do you think 1723 vs 1723s are in performance (dynamics,clarity etc) ? I am trying to decide between 1723 monitors + center vs 1723 S monitors + center.