A bruised head is better than a worse injury. Roll CAGES are even better, offering chassis rigidity along with greater protection all around. If you drive fast, I highly recommend a roll bar at least, but it has to be strong enough to do the job. I know of no sanctioning body that accepts Ti as a roll bar material, but they may exist. The usual stuff is mild steel or chromoly steel.
There may be one, and roll bars can be very effective. I saw a little Fiat convertible in a tire/auto shop once that was quite wrecked. However, the girl's dad had insisted the car have a roll bar. The roll bar saved her life, keeping the car from landing, upside-down, on her head. She escaped relatively unscathed, unlike her little steel-blue Italian friend.