The more flexible the barrier system the lower the risk to light vehicle occupants.
In their paper titled Feasibility of Predicting Light Vehicle Occupant Injury Disutility from Impacts with Road Safety Barriers, Burbridge and Troutbeck (2016) graphed the Acceleration Severity Index (a non-dimensional vehicle occupant severity indicator) vs stiffness of the system (graph pictured).  
But don’t forget, the selection of the appropriate barrier needs to take into account a raft of other factors, including; offset to hazards, horizontal and vertical alignment, severity of hazard, length of need, whole-of-life cost, consistency within a route, ability to maintain the barrier and the road/roadside and percentage of motorcyclists and heavy vehicles… just to name a few.