The Monarch is a notch higher model as such using better coating which should = optical gains such as; clarity, light transimission, etc. I have no experience with the Buckmaster so that is why I say "should".

Buy which ever power is best for your situation. The higher the mag the more critical the light becomes, meaning if it isa poor sight pictureat 7x it will be worse at 9x when comparing equals (same optics/models).