I don't look at them by horsepower, rather how much racket do they make, and how reliable are they.
I have had good luck with ISE (In-Sink-Erator) and Badger in the past, and for evaluation purposes, hp is a good way to judge the quality level within a particular brand name. I wouldn't purchase the lowest hp model, because it equates a lower strength of parts as well. The highest hp rating equates more disposal than a kitchen should have.
Look for noise ratings, and know that a stainless steel sink will be louder with a disposal than an enameled cast sink.
Beyond that, make sure that when you install a dishwasher with your disposal, you install a breather vent on the sink (the little chrome deally with the little vents in it) these vents must face the sink so that if you have a backup in the drain, the water will go into the sink instead of all over your counter, and possibly onto your hardwood floor (this always happens when you start the dishwasher as you leave for your 2 week vacation to Hawaii.)