I'm 99% sure it's the tackler that is concussed more often than not. However upright/high tackles are still the leading cause of concussion, both for the tackled player and the tackling player. If the tackler can be forced to bend, rather than hit upright, a significant number of concussions should be removed.
The stats were done per 100 tackles too, so whilst you'd obviously expect more people to be concussed by heads hitting hips, if they're all tackling lower, you'd still expect fewer concussions overall.