Here’s the referenced study, conducted by a company in Sweden, using 15 different riding helmets. Most helmets are designed to protect from impact, but very few protect against rotational forces. The ones with “MIPS protection”, which has been used in skiing/dirt biking/etc helmets for years, protect against rotational forces and scored the highest in the study. There’s also a chart that ranks them and includes their price; super interesting that price doesn’t necessarily reflect a “better” helmet.
My thought has always been that if we have to wear a helmet (I’ve literally never been on a horse without one, maybe I’m a pansy; I mean “have to” in the sense that it’s required at competitions) I can’t come up with a reason to wear one that provides anything less than the best protection. Like… if my head is going to get all sweaty and gross regardless, I’d at least like it to do its job well :lol:. And, as you can see from the study, they really aren’t all created equal.
Not saying anything that hasn’t been said already. I just find it so fascinating that they all meet “standards” but the protection level is so different. It kind of makes me wish they’d re-evaluate the standards. I just worry the standards lull people into a false sense of security, thinking a helmet is OK because it meets the standard, when in reality, the standard is just a baseline and there are other helmets out there that offer better protection.
To each their own, though, obviously. I get that it’s a personal choice. And TBI’s also happen to people while wearing helmets. I just love horse people and I want them all to be as safe as possible. 
http://mipsprotection.com/equestrian_helmet_test/