They're probably overall quite comparable as luxury hotels. For your purposes, the FS has the benefit of the downtown location and view of the Arch if that's worth the extra $100 for you. The RC has its brand standard Club lounge, which may or may not be important to you along with the better rate.
It's all a matter of your priorities.
It's St. Louis. Temper your expectations. There is a reason both hotels are less expensive than you normally expect.