yep, I'm with Eden on this... buy whatever helmet FITS you properly... lots of people wear helmets that move around as they ride... if you crash (heaven forbid!) it won't offer the protection you'd want. Fit is critical.
That being said, I ride with a helmet with 24 vents (I think it's 24... been a while since I counted!) because it's HOT where I live... the extra ventilation when I'm working hard helps tremendously...
I ALWAYS have helmet head after I ride... but what the hey... I figure it goes nicely with my tan lines, sweat encrusted face and all the other lovely attributes of cycling...
There is a fine line between "hobby" and "mental illness".