From what you describe, I can say with confidence that your string is too long. If you are left handed and when the string released the tip is pointed right, or toward you, the string is too long, if it is pointed left, or away from you, it is too short.
The Spintastics and Duncans will throw very similar if the strings are the appropriate length. Duncan strings are usually short, Spintastics strings are usually long. Long is of course better because you can always shorten the string but it is hard to add to a string. Tie a knot 2-3 inches from the end on the button side and test it before you commit to cutting it off. You can also adjust the angle of the release as alecto suggests. In this instance you would want to release with your palm pointed at the floor, or simply rotate the top in your grip to be pointed more toward the web between your thumb and forefinger, this should help.
Please post again with any problems you are having!