My friend showed me this article awhile ago, and I finally read through the whole thing.  I thought it showed an interesting perspective on the differences between a $100 helmet and a $400 helmet.

To summarize, the author of the article implies that Snell rated helmets may withstand higher energy impacts without breaking, but it does so at the cost of exposing your brain to higher G's, which is worse for your brain.  Thus, from a different perspective, some of those non-Snell helmets may be safer for you.

Anyways, I'd recommend this article, if just to learn a bit more about what goes on in designing a helmet.

EDIT: here's the URL!
http://www.motorcyclistonline.com/gearbox/motorcycle_helmet_review/index.html

The Snell standard has been updated since that article was written.  Probably a direct result of the article.

To be safe,... get one that's rated both...
Should say SnOT... ...