As noted, I'm surprised by this poll... I would think the thumb back towards you while keeping the gun in a safe direction would be tougher than slide release.
With your left hand (presuming you're a rightie) over the slide and your palm over the top your left arm should be at 90 degrees to the barrel of the pistol. Presuming proper muzzle control the pistol shouldn't deviate too far from straight in front of you, covering the target (where your pistol should alwasy be pointed). I believe this will get your pistol back into action faster than workign the slide stop with your thumb of your strong hand. My brother has fairly short fingers and his thumb does not reach the slide stop on his 1911 without adjusting his grip. This is a no-no as you always want to maintain a proper firing grip. If he waits until he gets his weak hand onto the pistol to work the slide stop he will be slower getting the pistol back into action than if he used the clamshell technique. Remember, "back in action" means a loaded chamber. You cycle the action using the clamshell and, if you have to, you can fire before you reacquire a two-handed grip.
Too, using the slingshot method requires you to tilt the pistol to the weak hand side, usually tilting the rear of the pistol to the left and down - that's just the mechanics of the human hand. You're off target and that much slower getting back into the fight.
Worse still, is the muscle memory in a failure drill. You're in a fight and shooting. BANG, BANG, CLICK... Now you have to do a failure drill. Tap the mag to ensure it's seated and now cycle the action. This is where adrenaline goes through the roof. Plus you have the weight of the hammer spring to overcome when you cycle the action. If you use the slingshot you are using an inherently weaker grip and using it in a stressful situation. This is where the fine motor skills will kill you. If your thumb slips off the slide as you're trying to cycle the action you've compounded the felony and your adrenaline will spike even higher as you try to get the pistol back in action. If you use the clamshell you are less likely to have a fumble when the chips are down.
For 90% of shooters it will never matter. But for those engaged in defending themselves, from bad guys or the zombipocalypse, it's vital that things work every time.