When I buy a helmet, my main decision factor is comfort. Having said that, I only pick from well known brands. As far as safety rating goes I go with the German TUV or British Gold ACU approval. Usually helmets with the aforementioned safety stamps are also approved by main racing organizations. Snell is overkill, DOT is a joke ( HD brainbuckets! ), plus I don't think their testing is proper ( ie. modelising real events ).
In my opinion comfort is THE most important factor, because if the helmet is uncomfortable ( even if slightly ) it'll draw the rider's attention from actual riding, which is very dangerous. Also, uncomfortable helmet makes the rider tired very quickly.
I've tried many brands ( Arai, Shoei, AGV, Suomy, Marushin, UVEX, Schubert, Roof etc. ) but finally I settled with the Force Ten from FM. That is the most comfortable helmet for my amorphous head
. Once I've ridden 25 hours straight in it without taking it off and I had no problems whatsoever. The finish is not on the same level like Arai's or Shoei's, but that's the price I have to pay for comfort.