The main difference between these speakers are how deep they play and the amount of bass they can convey.
If you are going to have a setup without subwoofer(s) in it, then the larger Tower S speaker is the way to go, Monitor S will be a little too weak for movies especially in the bass department.
Let’s assume you have one or several subwoofers. In this case it will depend where you can place the subwoofers in your room. If you are only able to place the subwoofers behind or to the side of the listening position, then you will likely have a better integration using the Tower S speaker as you can set the crossover frequency lower and thus it will be harder to locate the placement of the subwoofer(s).
If you have subwoofer(s) placed on the front wall where the speakers are, you really have no need for the large Tower S unless you are using subwoofers from other brands that are not as dynamic and tight sounding as Arendal Sound subwoofers are. Our subwoofers will meld into the soundstage of the Monitor S speakers making them sound like huge speakers. There is really no gain opting with the Tower S speakers in this configuration, well unless you simply like the looks of them.