IRH for me, simply because they manufacture a long oval configuration that fits my head shape. It was a big plus to see my helmet make it into the upper tier in the VA Tech report.
I have six riding-related concussions, two with memory losses. The helmets did not prevent concussions, but did prevented more severe injuries like skull fractures, especially the one where a rock actually punctured the helmet.
Here is an interesting report of shearing effects for anyone interested.
