A couple of clarifications:
1) DOT is the required US standard. However, helmets are NOT tested to meet the standard. Instead, the helmet maker "swears" that the helmet will meet the standard IF tested.
2) Snell standard is 100% optional on the street in the US. The Snell standard is actually very questionable as it allows for G-forces transferred to the head to be well in excess of accepted scientific thresholds that will usually result in brain damage or fatality.
3) The primary job of a helmet is to protect the head in a crash. However, it is not intended to deal with a 140 mph header into a brick wall. Rather it is intended to deal with the impact experienced by falling from riding/standing height to the ground. Pretty much any DOT full face helmet can handle this primary function.
4) The secondary and far more common function of a helmet is to protect the rider from wind, bugs, sand, debris, sun, cold, etc. while enabling greater vision and comfort and lessening fatigue and distraction, all of which leaves the rider much more able to effectively and safely operate the bike and maintain awareness, thereby decreasing the likelihood of an accident. Any helmet will do a decent job in a crash, but each helmet/rider/bike combo will vary in terms of increasing/decreasing accident potential.
IMHO, a helmet s/b chosen based on the following criterion:
1) DOT certified
2) Fit
a) Get educated about how helmet is SUPPOSED to fit
b) Measure your melon
c) Try on as many helmets as you can
3) Comfort - if it ain't comfy to ya, ya may not wear it. Even if ya do wear it, it'll be at best a distraction and at worst a detriment to your riding.
4) Affordability - Doesn't make sense to buy anything ya can't afford. On the other hand, it doesn't make sense to not find a way to afford something as critical as the best helmet for your particular combo of noggin/bike/riding. They're all different. Some are lucky enough to be a better match with lower cost helmets but most are able to realize significant benefits from the premium stuff while others go for the premium stuff only to find that it's a bad match.
5) Oh, and if it ain't full face, it ain't a whole helmet.
Best of luck!