When it comes to impact protection, they all meet the same standards and there is no way to prove if one provides more protection than what the CE and ASTM standards require.
So beyond getting something that is at least ASTM/SEI approved, it becomes a question of fit. And that's a question only you can answer by trying out helmets.
I have an IRH Olympian and have tried various other IRH styles and have found that all of them have been molded for more oval heads. On me, they gap along the sides of the head.
When it comes to show helmets, Charles Owens fits me much better and eventually I will get a GR8 to replace the IRH.
I also have a Tipperary Sportage for schooling and trail riding (so it gets used about 95% of the time). That helmet fits me perfectly, has awesome ventilation, and I like how it covers the back of the head better than just about any other design.
However, the Tipp Sportage is not a formal enough helmet for show hunters and show jumpers. It is plenty good for the XC phase of combined trials and 3 day events.