What are the differences between the two main brace positions? Why are there two different brace positions (pictured below) ?
(If there is a difference between the official and true reason, please state both. In many planes the reduced seating spacing does not allow the pike position (on the right), but I doubt this is the official statement of airlines.)
Are they equally protective ? If not, why are both recommended ?
What arguments are there in favor or against each ?
If available, statistical data and relevant links to studies and crash tests would be very welcome if they distinguish between the two positions.
One is for when the seat in front of you is in the way of using the other. 

The other is for when you can't reach a seat in front of you.

They won't both be relevant for the same person in a given seat. If you can assume the second position, the first one probably won't provide meaningful support for your head anyway.