Brace position - does it really save lives?
The FAA has a study on it you can find here the study is worth reading completely but the important parts of the conclusion seem to be, 


  Current Brace Position
  
  This position (head against the seat back,
  hands on top of the seat back) was evaluated for three common types of
  seat backs. 
  
  
  
  This position was only successful for locked-out type seat
  backs. For full break-over and energy absorbing type seat backs, the
  ATD’s arms pushed the seat back away, allowing the head to accelerate
  relative to the seat back, increasing the severity of the head impact.
  There was, however, no evidence that the ATD interaction with any of
  the seatback types resulted in hyper-extension of the shoulder joint.
  
  
  
  The “Forward” and “Vertical” leg brace positions permitted the lower
  legs to flail forward and contact the seat in front. This contact
  produced femur bending and compression (below criteria limits), and
  tibia injury risk in some cases. 
  
  
  
  The “Aft” leg brace position reduced
  lower leg flailing and prevented femur contact with the front of the
  seat frame. This position resulted in low femur and tibia injury
  assessment values; however, it is only achievable for occupants whose
  lower legs are long enough for their feet to firmly touch the floor.
  
  Alternate Brace Position
  
  To reduce the detrimental interaction
  between the occupant’s arms and the seatback, the current position was
  modified by placing the hands down by the lower legs instead of on the
  seat back. 
  
  
  
  This “Alternate” position was successful in reducing head
  and neck injury risk for all of the seat back types evaluated.
  
  Pike Position
  
  This position is currently recommended when the seat
  in front is too far away to support the head, or there is no seat in
  front, as in a bulkhead row. 
  
  
  
  It was successful in reducing head and
  neck injury risk in the row-to-row scenario, as long as the struck
  seat back has a relatively soft local compliance at the point of
  impact. When seated behind a bulkhead, the effectiveness of the pike
  position was dependent on whether the head of the occupant struck the
  bulkhead. “Head-path-reducing features,” such as Y-belts, prevented
  head contact at the typical 35-in setback.


So the answer is really, it depends on the position as well as the seats in question. You find a full explanation of what the various seat types are on page 2 of the study.

Perhaps the most interesting note is that they call for a potential change in the current brace position:


  ...and the current positions recommended in AC 121-24B may need some
  adjustment to provide an equivalent level of safety for all passenger
  seat back types.