Both switches have positives and negatives, some of which are points of view depending on how and why the firearm is being used.
In general the SL switch is considered to add too much bulk to the handgrips of some handguns, and require different grips to activate for extended periods, and the exposed switches at the sides of the firearm can wear quickly in some holsters and methods of carry.
The DG switch is more natural to use, and it is 'normal' for the light to activate when you grip the handgun, as it desired by some users. It is less susceptible to wear and being damaged and easier in some cases to operate the switch and hold it constant-on for longer.
With any WeaponLight the individual user is the most important person to determine what works and doesn't work for them, based on their training and experience and the uses they are requiring light for. One of the best ways to determine the best choice is to try out a variety to find out. I appreciate that this can be expensive and time consuming but it can be well worth it based on the feedback I've heard, and what I read online.
Al