The Highlifter has slotted rotors. Some people say that if you take them in mud and sand, the pads wear out way fast due to the sand and dirt getting trapped in the slots and grinding away at the pads when you brake.
The Xtreme doesn't have the slotted rotors so they won't trap dirt and sand in them.
What i think is that, if you ride mud a lot, the Xtreme is the best one. Other than that i would go with the Highlifter.
Hope this helps...