The STS is a great speaker, no doubt. You certainly can't go wrong there.
But, just to play Devil's advocate...
I ended up choosing the Ones over the STS's because I had planned on getting 2 subs. The thinking being that Ones plus a sub would give me the functional near equivalent of a Mythos with a built in sub, but with more flexibility of sub placement (since the optimal position of the sub(s) is rarely the same as the optimal position for the front speakers). The 2 free standing subs also gave me more headroom than the built in subs of the STS.
In addition, a potential advantage, however slight, of the Ones over the STS is that it uses the same drivers as the Eights. Since I was going with Eights as the Center and the rear surrounds, as well as the RSS II's as side surrounds, I wanted the front's drivers to match each of those speakers as closely as possible. (The STS's drivers are a little smaller and closer to the Fours and the Nine. The drivers of the Ones are similar in size to those of the ST) I also had to spread my cash over 7 speakers and 2 subs, so the cost/value considerations also came to the fore.
In Chopin_Guy's case, however, he has a brutal sub already, so I think the STS would fit in nicely and give more ooomph than the Ones (especially in his open room). However, I suspect the Ones with a proper crossover would be nothing to sneeze at, especially when using an Eight as the center.
Just my two cents.