Most helmets are made to the same safety standards and so, in theory, Offer the same protection
But you definitely want a helmet which protects the back of your head well, for when you go over backwards.
The giro xen is an enormously popular helmet amongst trials riders, and for good reason too. It's an awesome helmet, covers the back of your head and is ridiculously light, very comfortable to wear.
They're not all that cheap, but definitely the best helmet I've ever worn