Don't buy a helmet without trying em' on. AND don't buy into the "Cheap Helmets" work just as well BS. They may meet the "Minimum" required by law but they are by NO means better, just cheap.
Best helmets in terms of safety, quality and fit? Arai. Next best, Shoei, Suomy, and some others. Then you have all the rest that promise safety and features on a budget, HJC, Shark, Bell, KBC, ICON, Scorpion*, etc...
*Scorpions use the same cheap plastic shells as the other budget helmets but they also have a lot of nice features and a good push from the manufacterer.
All the full face helmets you'll find on the racks at most dealership will meet DOT spec which isn't much comfort, then you have SNELL which takes the requirements for safety and measured energy transmitted to your noggin a bit higher.
While many of the inexpensive helmets are rated both SNELL and DOT most just MEET the standards, while your pricier helmets far surpass the standards through the use of stringent quality controls, better materials, and better design.
So I recommend an Arai, or a Shoei. Simple. And just a couple of months agoI bought the New Arai RX-7 Corsair "Tommy Gun" and a Shoei RF-1000 Diabolic. After working on the shop floor for a year and watching the helmets that come in for repair and the problems people have with their helmets as well as seeing crashed helmets here and there from the race team and customers, I am an even bigger Arai Fan than I was before. We had guys crash and whack their heads really hard in Bell, and Fox offroad lids that ended up with bad concusions. While we had a couple of guys in Arai's tanked it HARD smacking his head into the walla few months ago, (Maggies Boy) and while he was loopy, his melon was intact no concusion, no worries, the other slammed it into the asphalt a couple of weeks ago at a track day event, again impact was pretty severe and he just walked away...
So it's up to you really, but that's my .02. One other thought, you have to try the helmet on before you buy, no mail order on Helmets, you don't know where it's been.