From what I have read the testing is very similar with only a couple of minor differences. I was going to buy a LS2, but they just didn't fit and I didn't care for the Shark. I wasn't sure about ordering, but took the chance
DOT has proven it can be a safer helmet spec than SNELL. In real life accidents. A DOT certified helmet was giving riders a better result than SNELL helmets, in many cases.
It's a long story and it depends on the particular DOT helmet. But SNELL was designed around Automotive Racing needs where you are strapped in to a fixed location and your head is extremely likely to hit the same hard part in the same spot more than once. This is a virtual impossibility in a motorcycle accident. This simple fact should change everything. But for SNELL, it did not up until recent years. Instead, they spent money on convincing the motorcycle owner that SNELL was the pinnacle of safety. And nearly everyone took their word for it.
The whole idea of a helmet is to 1- reduce the G-Force to the brain on impact. This is done, primarily, by allowing the helmet to crush, absorbing a great deal of energy before it reaches the rider.
2- The helmet needs to keep objects from penetrating. This 2nd aspect is very important in an automotive accident. But not nearly as much in a motorcycle accident. A hard shell (SNELL) helmet is great in this aspect. But it sacrifices objective #1 to get there. Objective #1 is most important to a motorcycle rider who can safely give up some of aspect #2 to achieve better impact protection.
Making the shell very hard is a good thing on a car application. But it is not good for a motorcycle application. A hard shelled helmet reduces the ability to control G-Force to the brain. SNELL, even after years of evidence that their approach to motorcycle helmet safety needed to be completely revamped, continued to ignore the issue and instead, spent more money on marketing their safety. Especially troubling in the face of hard facts.
The European ECE standard threw out everything and started from the ground up with motorcycle safety in mind. With proven results, I might add. The DOT standard was much closer to the ECE regs than SNELL. Unfortunately, the DOT specs also allow for very cheaply made helmets. But you can easily distinguish the difference in a quality DOT helmet.
After years of pressure, real world motorcycle accident statistics and lab room testing. SNELL has finally been forced to see the light and are making some worthy changes. They might be better off just going with the ECE testing methods. But they have not gone that far.