The main benefit of using the slide release lever is that it is substantially faster. In a fight, if your gun is empty and you need to reload that means you’ve already fired a number of shots and it wasn’t enough. So it goes without saying, time is of the essence. Faster is good.
The main benefit of racking the slide is that it’s nearly universal. Whether you’re right-handed or left-handed, whether you’re using a 1911 or Glock or Kel-Tec, pulling the slide back and releasing it is going to work.
You will often hear that racking the slide is also “more reliable” under stress. This is a myth. As soon as you hear someone starting to talk about gross motor skills and fine motor skills, you are officially justified in tuning out. Both racking the slide and hitting the slide release are fine motor skills.
Fine and gross motor skills are terms from child development and child psychology. They are pretty clearly defined.
A gross motor skill is one that uses only major muscle groups, like arms, legs, & trunk. Walking is a gross motor skill.
A fine motor skill is any movement that requires smaller or more precise muscles. Pointing, grabbing, even squeezing with your hands are all fine motor skills. Basically, anything that uses your fingers is a fine motor skill.
So:
•Hitting slide release lever = fine motor skill
•Overhand racking slide = fine motor skill
•Slingshotting slide = fine motor skill
•Banging head against wall after hearing this debate the 1,000th time = gross motor skill
Sources:
•babycenter.com
•Developmental Psychology Newsletter
•Wikipedia
What it boils down to is this: if you have enough “motor control” under stress to press the trigger properly and press the mag release button properly, then you are equally capable of hitting the slide release lever if the lever is properly sized and located.
So the long answer is: If you can reach the slide release lever comfortably without a major shift in your grip, it is faster and less fumble-prone than racking the slide manually. If you cannot hit the lever reliably, then racking the slide is a better option.