It actually seems like both of them would rather not rate them on stiffness but were forced to because of questions. So its a matter of opinion it seems...
Wrong. As I have stated before, Union prefers not to get into flex ratings for a number of reasons (people misunderstand the whole flex/responsiveness thing, changes between models from year to year, flex of different parts of the bindings, etc.), but the facts are quite clear:
- SL and MC have fundamentally the same baseplate - Atlas Stage III. However, MC is cored out - hence, not as stiff. Stage II baseplate on the Force is 'beefier' and stiffer than either.
- SL and MC highbacks are very, very similar - carbon is used in the MC to reduce weight, not to increase stiffness. Force highbacks are quite different, but at least as stiff in most respects.
Therefore, overall stiffness: Force > SL > MC. Which is how all our shops have marketed them - and incidentally entirely consistent with my personal experience.