The answer to your question of which standard is better is not simple; there are advantages and disadvantages to both, and there are also other standards to consider, such as BSI or ECE. Snell and other organizations test for some aspects not included in the DOT standard, such as peripheral vision, chinbar impact absorption, faceshield penetration, and how well the helmet stays on your head with the chin strap done up. At a minimum, purchase a full-face name-brand helmet with a DOT sticker. You can be fairly certain it actually meets that standard, whereas you take your chances with something you buy for $20 at a flea market, DOT sticker or not. If you are concerned about coverage, impact absorption, certification or some of the other tests mentioned, research the various standards and choose the one that you think best meets your needs. I always recommend that people purchase a helmet that meets DOT standards and is certified to at least one additional standard. That way you can be certain the helmet meets the standard it is claimed to, and has the appropriate coverage area and other safety features that the DOT-only helmet may not have.