sycamore,
The Cheyenne was cancelled after three of the 10 prototypes crashed, said to be for rotor stability issues associated with its unusual gyroscopic control system. The last crash involved a rotor-fuselage contact that killed the pilot. As a 22,000 pound helo with about 4000 horsepower, it could dash at 220 knots, and had a 10 foot prop on its back end.
The S-67 Blackhawk was developed by Sikorsky in 1970 after the Cheyenne production was cancelled, the two aircraft were not in direct competition. I have about 5 hours in the S-67, including several aerobatic maneuvers. It was a very fine helicopter.