I've heard variable sized arguments usually get icky in coding?[/side thought] If this is implemented, I think eyup's solution is the best so far, by using 1 or 0 to differentiate between hard/tween.
However, we do have to remember that the UI will be implemented soon, so this wouldn't really make that much of a difference, unless you're someone who gets emotionally attached to manually typing out the storyboard. It's just 1 or 2 more arguments, and I think the current way (ie. explicitly saying what it's going to do) is better.
Hard moves can be down presently by repeating the (x,y) coordinates twice, I think.