Not sure if we're going to see Mr L stop by here, but buried in this deep thread Nick talks of 100% being a more efficient rotor speed below Vy due to S76 wide chorded blades, which were designed for high speed flight. 107Nr is better for tail rotor effectiveness and low speed handling, if I remember correctly. Certainly in the 139 the bump up to 102Nr whips the tail end into line. The change in tail rotor effectiveness is quite noticeable.