Slide release vs. racking the slide back
Is there a “proper” way?
Current SOP is hand-over-slide. This is the fail-safe because it can be used to perform other functions, too. (For instance, you use this same movement to clear malfunctions.) The other alleged virtue of the hand-over-slide KISS methodology is that learning to do things one way means you won't debate which slide-racking method to use when time is critical. I think that argument is silly, but others do not. Thus the primary virtue of HOS is versatility. You can load that way, unload, and clear malfunctions.
Using the slide stop to reload is faster, but only if the slide is already locked open. Thus this operation does only one thing. It drops the slide.
Slingshotting the slide is possessed of its own virtues, too (at least if you're an Israeli commando, something I point out only half in jest), but it's not a method I use much at all and in fact practice little.
The question, then, may not be so much which is “proper,” but which is proper under what circumstances.
When pursuing peak reloading speed, I use the slide lock lever.
When initially loading a pistol, or clearing a malfunction, I use HOS.
My goal is to be versatile and to flow smoothly from one technique to the next, whether it's reloading as conditions and equipment dictate, or transitioning from one weapon to another.