The biggest difference is the way that they handle. The HPAR is going to be heavier and slower swinging, the Mono is lighter and quicker handling. The Mono may have different heights of ribs. Pick the one that fits you best and handles the best according to your shooting style. A faster shooter probably would choose the Mono, while the more deliberate shooter might prefer the HPAR. Both are built like tanks, and both pattern well if the barrels have not been messed with. Both tend to show bluing wear. Doug Braker did a terrific job of restoring a worn HPAR for me by nickling the receiver. They are both great guns with quick lock times, nice trigger pulls, good reliability and ease of repair/servicing. Prices are soft right now for both guns, but they are worth every penny for what you get. Shoot both before you decide. Bob