I was told by the lady at the counter who helped us pick out our helmets that snell is a higher standard than DOT. Not sure if that's true,
Here's a snippet from Revzilla that I just came across at this URL.
http://www.revzilla.com/motorcycle-helmet-guide
"What kind of safety ratings are there?
There are two main types of safety ratings: those required by law in a specific city, state, or country, and those submitted for testing to a third-party organization. The former, like DOT in the United States, ECE 22.05 in Europe, or AUS 1698-2006 in Australia, are largely voluntary standards. This means that while a certain level of protection is required, no testing is needed in order to produce a helmet. DOT will eventually get around to testing every helmet that is on the market, but until that happens, you are trusting the reputation of the manufacturer to meet the standard.
The second group is filled by those like SNELL or SHARP. A manufacturer will submit a helmet to these third-party testers for approval in order to receive this certification. The helmet then goes through a robust testing process for a random sample of each shell size, from several batches of helmets. If they all pass, the manufacturer must then pay to carry the SNELL or SHARP certification sticker on their helmets for each model produced. This acts like a badge of honor and you can rest assured that several clones of your helmet have already been subjected to cruel treatment and passed the test. Obviously, your exact helmet wasn’t tested.
Which safety rating is the best?
Different standards focus on different kinds of impacts. Some focus on puncture protection while others focus more on energy absorption. DOT and SNELL fall into the first camp, testing motorcycle helmets by dropping pointed weight on the sides, top and chin of a helmet to test for penetration. Other than a small chip or crack at the point of impact, these shells tend to look virtually unharmed after testing. The second, like SHARP and ECE, concentrate more on transmitting the energy administered upon impact throughout the entire helmet. These helmets tend to look more maimed after testing, but that is the point. The helmet absorbs the energy, breaking down and spreading the force across the entire shell. These tests also introduce a few more oblique impact results into the equation.
Which is "better?" That's been the subject of fierce debate. Here's the rub: Tell me what kind of crash you're going to have and I'll tell you which safety rating is better. Since none of us know in advance how we're going to crash or what our head is going to hit when we go down, it's a complicated decision. People come to different conclusions on which safety rating they trust. That's why in the beginning we focused on fit first, and safety ratings last. The one thing that's certain is that a helmet won't work right if it doesn't fit properly, no matter what sticker is on it."