Generally, the specifications to start comparing are torque and pullback. These two will determine your max drill depth and bore diameter, and give you a good starting point for gauging the capabilities of a rig. Note that while torque specifications often refer to drill power, drill speed can be more important depending on your application and drill method.
Most equipment manufacturers have spec sheets available online that can be used for comparison. A word of caution: Be sure to examine the specifications carefully as some spec sheets use different units of measurement (such as inch-pounds vs. foot-pounds of torque). In the final analysis, the balance between available power specs, equipment size and final price can narrow your search to the point where the best options are clear.