But which method works best and more reliably?
I think it very much depends on several factors. This is why we teach both methods and let the students choose which fits them the best.
For example... Someone who does a lot of shooting with a Sig and carries a Shield as a CCW might prefer to rack the slide as opposed to use the slide stop/release because the two gun's slide stop/release is in different locations. The same is true for many others who use multiple different handguns where the mode of operation is different. With most guns, racking the slide works regardless of type.
Many left-handers choose to rack the slide because only a few modern handguns have been designed with ambidextrous use in mind and many slide-stops (especially Sigs) are difficult to operate left-handed.
Competition shooters are generally concerned with maximum speed and efficiency, and for pure speed and efficiency (barring left-handers with non-ambi pistols, etc.) the slide stop/release will likely be faster and more efficient with appropriate training and technique.
If one is using a similar platform (M&P/M&PC/Shield, etc.) using the slide stop will likely increase speed at minimal sacrifice to reliability.
I have seen many people "miss" the slide (racking) during a reload...this requires them to re-acquire a grasp on the slide and re-rack. I have seen people "miss" the slide-stop/release, but there is significantly less movement involved in re-acquiring the lever and making it work. In either case, delay is somewhat minimal, because the biggest delays I've noted in reloads come from accessing, indexing, and inserting magazines...and failing to remember to send the slide forward (rack or release) at all. There is probably slightly more "reliability" in racking the slide, but I feel it's overblown and overshadowed by the weapon-platform-consistency argument. I personally feel that the increased index afforded by the strong hand already gripping the gun or the support hand moving to a traditional grip provides very positive actuation of the lever and thus I prefer it.
I have been trained in both methods by some of the best instructors in the world...I use both depending on the circumstance, but very much prefer using the slide stop/release. This has much to do with ingrained training, but it is also faster and more efficient if done correctly and with a platform which is familiar (or consistent with the familiar) to the shooter.
Overall it's such a minimal difference that I wouldn't push a student (other than a competitions shooter) either way...there are much bigger fish to fry. The whole hubbub above stems from my somewhat OCD pet peeve of people stating absolutes at inappropriate times and with a lack of experience and knowledge. The use of defensive firearms (and the Shield was designed for that purpose) is a life-or-death issue. For such matters, having people on-line with little to no experience and minimal knowledge passing opinion, rumor, and misinterpretation of information as proper technique, let alone the "only" proper technique is simply irresponsible.
As I said before...You wouldn't read a manual to learn to drive a car and I strongly recommend seeking out a competent instructor to learn self defense techniques. If you wish to increase your knowledge beyond the basics, seek out several instructors to the point you can contrast teaching styles, techniques, and methods, applying what works best for your lifestyle and circumstance. There are so many variables involved from clothing, weapon type, environment, attitude, mindset, physical ability or disability, strength, and even so much as height and weight, that no one "size" fits all when it comes to training and technique.
