Thought I'd share.
So I asked why and he said the longs ones make a large arc, easy to pick and parry by reflex. Just grab the long line and you've basically disarmed your opponent, but the short one (according to him) produces 80% of the momentum of the big one, cannot be grabbed and is a lot more difficult to spot quick enough to parry.
your friend makes sense. 90% of the time I have my keys in hand, I'm in the "short" position shown in the pic you showed anyway. The ability to stretch it out to 30" is just "cool" I think. the reality of stretching it out in a self defense situation is slim; but lets say there were multiple attackers who were unsophisticated. perhaps no one would be brave enough to take their chances as you whirl the thing around? I don't know. I'll tell you what, I can swing this thing so fast and so quick using "rock sling" technique I'm guessing your arm will be in a huge world of hurt if you stick it out and let this thing wrap on you. just saying.
You apparently have in mind that you will flip one of these balls (with steel core), on a short piece of paracord, at your attacker.
It's worth remembering that in all the martial arts approaches that use weapons, skills and techniques have developed and evolved over very long periods of time.
The last quote makes the most sense to me because I believe the slungshot back in the olden days wasn't used the way you guys describe it, like some kind of flail.
Instead, the most effective way was as a weapon of surprise.
Basically the slungshot man simply threw the ball/shot at his victim and used the line/cord to retrieve the weapon for another go if necessary (or used it as a flail for the continuation of the attack).
As a weapon of surprise it was most likely more common with the shadier characters about town which is probably why it was banned just about everywhere.