Other than online folks, or stories at the rink, I've only seen one head injury. It was an ABSOLUTE beginner who toepicked over his rental skates, and fell face first into the ice (he clearly had no "catch myself and break my wrists instinct"...), I think this was his first time on the ice. He smashed the top of his forehead into the ice and wash GUSHING blood. I don't know the severity of it (foreheads tend to bleed A LOT regardless of severity, just because there is a lot of blood there), but he did have a concussion. He left in an ambulance, and returned the next week to take classes, where the skate director told him she would give him a full refund and the next set of classes free, but she wouldn't allow him back on the ice for 4-weeks after a concussion. I think this, along with another woman who sued over a broken wrist, is what killed our adult LTS. The manager now has to approve adults to enter LTS, he thinks they are too much of a liability.
I think head injuries are rare, and if you can only wear one pad, I'd pick a hip one. I worry greatly about slamming my head though because I don't fall well, every fall is a bad fall, because it only happens about once or twice a year. I also have some spinal cord issues, so while I probably shouldn't worry more than anyone else about ice skating, I do.