my 2cents
If constructed properly the plastic or wood retainer systems are not hard on cars at all. It's when you have ill constructed turns where the barriers meet that without the right type of deflection barrier (flapper) will be hard on cars. Also the seems where the barriers meet if not secured the right way is also hard on cars.
Road rails however do not offer any protection from cars hopping a barrier this is important especialy with a high speed straightaway. And what I have expierienced is the road rails are brutal on tires IE chunking I would rather bounce of a wall then go over a rail and chunk a tire or two.
Having had both systems I have not noticed a real difference as far as breakage goes but I have noticed a considerable difference in tire chunking so my opinion on road rails is if they are your barrier of choice consider using rubber tires would be one suggestion.