That's a tough one - sorry. To a great extent I think it's a matter of construction and maintenance. There are wooden roller coasters that have been in continuous use for 50 or 60 years and there are steel rides that had to be taken down after 10 years,
There's also the problem of how you define "safer." Many wooden coasters, while giving a heck of an exciting ride, don't have warning signs that tell you to not ride if you have neck or back problems like most of the modern steel rides do. And the wooden rides are NOT gentle, they'll lift you out of your seat, slam you down, throw you against the sides on curves, etc. But apparently there's something about steel coasters that make the ride inherently more violent.
It seems like, when I read about injuries or deaths on a roller coaster, wooden coasters have more incidents where some idiot tried to stand up or did something else that allowed him or her to fall out of a car or strike part of the structure. With steel rides, it seems like there are more mechanical failures that cause injuries. At least that's been my take on it