Both are fantastic bows.. I own a stickflinger and one of my best buds owns two heartstoppers, os have a fair amount of experience with them as well.
Duane (Heartstopper) arguably makes one of the best looking 3pc LB's in the industry. They are quick, look great, very well balanced, and dang can he pick some wood combos... He really is a master bowyer..
That being said I feel that the stickflinger is a smoother drawing/faster shooting design. Brian too makes a beautiful bow, and the grip on the stickflinger really just melts in the hand. I am a big TallTines fan whether it be RC, stickflinger or tundra and still have one of each. They are just great performing, extension of the hand type bows.
The grip on the heartstopper is my main complaint, I prefer some sort of locator on a hybrid design LB, and his grip is relatively straight. That doesn't mean I dont shoot my buddies well and want one of my own! I like a straight grip on certain bows (D/R longbows) but for whatever reason like a locator on hybrid styles, but that is a personal thing. Maybe he can make a locator for me?!
Hope that helps