The length of the cord and weight of poi head are relative to each other.
Which means that.
Short poi cord, heavy poi head = Easy fast but precise isolation
Short poi cord, light poi head = Difficult for the arm to move around such an uncertain point.
Medium poi cord, heavy poi head = I think this is ideal for really nice isolations if you can handle the weight at all, otherwise it will fall.
Medium poi cord, light poi head = Less difficult then short poi cord, light poi head, because you get more time to move around the point. You do however loose some spinning momentum.
( Won't cover the long poi cords. )
So there you have it. That should be accurate, it is most probably just my personal preference. So perhaps anyone else begs to differ? If your can't handle the heavy poi just switch that for medium and your on your way to make nice isolations.
Everyone I've helped thinks it's easier to isolate heavier heads. But one should also know that to heavy is more likely to fall as well due to the small amount of momentum that goes into the head.
Play!