Up until recently my experience was that there wasn't much difference between helmets in terms of ventilation. Then I found the Kask Protone Icon. Noticeably cooler. Huge vents all over. It is so much better than my previous helmets that I find I need something on my head in cooler weather to keep from getting chilled. Looking forward to a much more comfortable summer riding season. Unfortunatley it is not an inexpensive helmet.
As others have said, it's best to try helmets on. There are different shaped helmets for different shaped heads. As an example, every bontrager helmet I've tried on was horribly uncomfortable. It's not that they are bad, they are just not shaped for my noggin.
And if you don't mind odd colors, you can often find super discounts on helmets that didn't sell.