Finding a helmet that's comfortable is important because you're more likely to wear it 100% of the time (even the crappiest helmet is better than no helmet at all). The challenge is finding a local dealer that has a decent selection to try on. For every helmet your local dealers carry, there's probably 10-20+ more available from Internet stores. My personal feeling is I'd never buy a helmet without trying it on first. Having done that, I'd shop around online for the best price once you found the one you want.
DOT vs. Snell is a very contentious issue, both here on this board and in the motorcycle community in general. The views are numerous but the facts are pretty straightforward: most DOT-only helmets subject the head to fewer G-forces in a collision than Snell-rated helmets. Pretty much all of them will go a long way toward saving your life; the difference in those G-forces, however, will help determine the quality of life after the accident.
If you've taken the MSF's Basic Rider's Training course, you'll remember that your brain is your most important piece of safety equipment. Plan and anticipate what can happen, practice your quick stop and quick swerve maneuvers, and hopefully you'll never get into a situation where your helmet comes in contact with the pavement. Assuming this, finding a helmet that's comfortable and has the features you're looking for are very important. There's really no way to find out until you try one on.
Okay, the speech is over and I've put my soapbox away ... on to your specific question. So far I've only owned one helmet, an HJC CL-MAX modular/flip-front. I've not noticed any difference in airflow with the vents open vs. closed, so I'd give it an F for venting. However, I can always open the face shield to let air flow in a bit better (that can be said for any full-face or modular helmet). One thing I've noticed is the HJC creates a slight hot-spot right on the front crown of my head. Wearing jimi caps and do-rags haven't helped, only the shelaclava from Aerostich prevents that sensation. I don't know if this is because of the helmet shape or it's lining material (which isn't removable as far as I can tell). The problem is, this doesn't kick in until I've been wearing the helmet for a few hours, not something I could have determined trying it on at the store. For some great helmet reviews, go to webBikeWorld