The mystical has more deflex/reflex, i.e., there is some deflex coming off the riser and more reflex in the middle of the limb working toward the tip. The back of the Hill riser is straight and the limbs come off the riser straight and are usually set back a little, usually about 1-inch, toward the tips, or 1 inch or so toward the belly if it's a string follow bow.
The locator grips on the bows are similar in that they are both mild, but they are a bit different. The Robertson is usually a little deeper and extends down the riser a little further. The Hill is just a little indentation under the arrow shelf tapered into the heel of the riser. The bigger difference is the depth of the grips. The Hill is 2 inches or more, the Robertson closer to 1 3/4 inches.
Their shooting characteristics are similar, but they each have a feel unique to their style. All other things being equal, the Robertson is a little faster. I like them both.