the foil and the SPK have the exact same flex rating, as mentioned above. Comparing the two, i would think that the SPK doesn't have as much all-mountain capabilities as the foil because of how much padding there is. Boots can have the same flex, but can feel totally different depending on how much padding there is and where it's located. Also, the vertical thanggg mentioned above... i'm ASSUMING that would diminish from all-mountain performance because when you turn, you transmit energy from your boot to your ski by a forwards-oriented pressure. if that makes any sense... so if your leg is straight up, and not leaning forwards, it would be more of a challenge to initiate a turn. anywho, foils are still pretty light in comparison to other boots. if you ski all mountain, i would suggest the foils rather than the SPK if you're looking into a salomon boot. i haven't ridden the SPK's so i might not know exactly what i'm talking about. someone correct me if i'm wrong.