I guess I should chip in here.
Answer: Something else...probably...
I worked for Motorcycle City for four years before its demise in one of their clothing & accessories departments as a manager. During my time there I learned a lot about helmets, mainly from the courses they sent us on at Arai.
While its easy to say one make is better than another, one fact holds true above all others: The one that fits the best offers the most protection.
There are other factors such as the materials used but all I will say there is leave alone anything made of plastic or made in two halves and bonded together, I'm hoping I don't have to explain why. There is one slight problem in that some bonded helmets are covered in a printed plastic film which covers up the obvious join, but there's not a lot you can do about that.
In most cases you do get what you pay for, and fortunately Arai and Shoei do still seem to hold the best quality records which is reflected in the priices charged. Its good to reinforce the fact that Arai and Shoei's R&D is the best in the business.
I could mention as an example that Arai use thicker material around the centre of the helmet than is needed, thinning it out towards the base slightly and leaving normal thickness at the top, the reason behind this is that a helmet is likely to hit the ground around the side or top, and unlikely (think about it) to hit around the base, the result in a helmet with a low centre of gravity making the whole thing feel lighter when worn.
Without going on and bore everyone to death, I wanted to offer the following advice:
Try it on for size AND SHAPE.
One make will fit better than another.
Different shapes exist within the ranges available from the same manufacturer
Don't set your heart on a helmet, it might not offer the protection if it doesn't fit properly.
Go to a shop with staff willing to spend time with you to get things right. (I would spend as long as it took with all my customers)
Some helmets can be built to fit.
If you know you are a 57 medium, and only a 60 large fits, it is the wrong shape!
And above all....
You have one head and no second chances, don't blow it!