I haven't wore a pair, but basically there the same. The big difference in HMK's vs regular snowboard boots is that the snowboard boots will be stiffer, or have more ankle support. This could either be bad, or good depending on what your looking for. I myself were snowboard boots, probely mostly cuz I snowboard and haven't found anything better.
sizzler - my highmark boots just arrived 2 days ago. i am going riding tomorrow and i will let you know how they work out. the differences between HMK boots and regular snowboarding boots is that there is less of an angle for the ankle support. also, the HMK are designed to form around your foot after several rides for best comfort. i got mine for $190 shipped to my door. basically the HMK boots have all the best features of snowboarding boots but are designed for sledders by sledders.
Most snowboarding boots also have a inner liner that forms to your feet. The point is that they (snowboard boots) have more ankle support the HMK's, plus the angle of the ankle on the boot is the shit......unless all you do is sit and ride...?
i wore my HMKs today and loved them -
they are very warm and comfortable. after using convential winter boots like sorels, they take a little while to get used to but they are worth it.
i would much rather wear the HMKs than regular board boots - stiff snowboard boots suck to walk around in. the HMKs offer a little bit more flexablity and will break in after a few rides. but whatever - it is your choice. both boots are basically in the same price range. i would pick the one designed for sledding. did i mention that they look damn good too? 
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