I'm sure someone has mentioned already that you need to buy the best you can afford, depending on how much you value your head.
And you absolutely MUST try them on.
Personally, as someone else mentioned, I'd go with a full face one, because even if you have harnesses, and you have a shunt, all manner of sh*t will hit your face, including, possibly, the steering wheel etc.
Also, full face has the benefit of being able to be used if you go karting.
Get yourself a balaclava as well. You might look like a tool, but you can wash the balaclava much more easily than you can the inside of your helmet, because no matter what anybody says, you will get damn hot wearing it for any length of time.
I have a sparco formula, which wasn't too expensive, but I didn't feel, given how often I was likely to do trackdays, that it was too cheap either. And it fit me well.
Just my two pennies worth. Feel free to disagree :-)