Union generally does not like to give flex/stiffness ratings for their bindings. Among other things, they point out (correctly IMO) that it is not just overall flex, but also where the binding flexes - e.g., the Contact Pros have a pretty soft highback but a very responsive baseplate (similar to the Rome 390s).
In addition, flex on Union bindings changes from year to year - e.g., the 11/12 SL had carbon in the baseplate, while the 12/13 does not.
In very general terms here is a spectrum of Union binding stiffness from my experience (i.e., not including the DLXs, etc.) from Soft to Stiff:
Contacts < Atlas/SL 12/13/Contact Pro < MC < Atlas Team/SL 11/12 < Force (up until 12/13) < Charger
Union Inhouse might be along shortly to add/correct the above.
Ok. So I think my idea of stiffness/flex, response, etc. vs riding style/preference is not as generalized as I had assumed. The more I read and learn, a lot of this is a matter of rider preference. It sounds like the best thing to do is try to demo as much gear as possible. Sorry for the noob inquiry guys, still trying to wrap my head around all this tech, which is constantly changing!!!