Downn2,
Everyone has made pretty much the same point, get whatever is going to fit your head. Easy....and it is the most important consideration. If it doesn't fit right, it will wiggle around on your head at freeway speed, if too big, or, it will crush your cheeks, ears, forehead, whatever, if too small.
There are however other considerations to make, as well. I learned a great deal about my helmet before purchasing, and it ended up having more benefits than I even imagined looking into.
I wear an Arai, RX-7 Corsair. Yes, one of the top three most expensive helmets on the market, but the benefits, IMO, pay for themselves.
Vents: Very important, and I haven't seen a helmet that really competes.
Comfort: The most comfortable helmet I've ever worn! Hands down!
Color scheme selection: More than enough choices to satisfy your taste.
Ease of use: This is where I ran into problems, initially....
Changing out the faceshield on this thing was a night mare in the beginning, and my experience with Arai's customer service was less than favorable, until I really made alot of noise, threatening to boycott them all over the internet. Amazingly, I got steller service with them after that. My wife's Shoei RX-1000 is ten times easier, but......when it comes to wanting to open it, or just crack it open at a stop light, the Arai is great. The Shoei has a "ratcheting" type of action to it, and isn't the easiest to open, but the Arai has a full range of motion, smooth and effortless.
Aerodynamics: Between my Arai, and my wife's Shoei, I like the aerodynamics of the Shoei best. If you will be riding two up, I would recommend getting your pillion the same helmet you where. The reason I say this is because when my wife rides with me, there is some weird ass tugging feeling I get on the top of my head. The air flow over two different helmets is odd. If you think about it, with me being in front, and her behind me, I shouldn't feel any different drag because I am hitting the air first, right? Well, for whatever reason, it makes a difference, and I don't feel it when I ride alone. I suspect that two helmets from the same manufacturer will not do this, but I have nothing to base that on. Completely a guess.
Warranty: Arai has the best on the market. A full 5 year warranty. No one else offers that. My issue with the changing of the faceshield on mine was a warranty issue, and they rectified the problem quickly by sending me new side pods and base plates for both sides. Since then no problems.
Well, there is my 2 cents. Good luck, and be picky when choosing. Oh BTW, see if the dealer where you get your bike offers a discount on riding gear when you get your bike. My dealer did. It saved me $130 on the helmet alone.....
Aaron