Jorge, I have a HyRoad and a Grease Monkey. Both have similar core dynamics and cover stocks. I find the Grease Monkey to have similar length but marginally stronger than the original HyRoad. The Grease Monkey Whack is somewhat stronger than the original Grease Monkey with a very similar roll and shot shape but, again marginally stronger than both the original Grease Monkey and the HyRoad. Each of these are excellent pieces that have a place in every league and tournament bowler's bag. When the HyRoad and Grease Monkey/Grease Monkey Whack shot shape is working, you really need to own one to keep up. If I were planning to keep the HyRoad in the bag, I'd add the Grease Monkey Whack as the step-stronger ball (as the original Grease Monkey, while marginally stronger than the HyRoad, is really close to the HyRoad). I'm of the opinion the Rocket and the Grease Monkey Whack are close in core dynamics, coverstock and overall reaction. To me, the Yeti line are closer in core dynamics, reaction and shot shape to the Marvel and Optimus lines. One bowler's perception, YMMV!