Attitude control loop investigation

Hello every body
I want to compare open source autopilots control loops.
I have simulated control loops of pixhawk, paparazzi and open pilot attitude control loops in simulink.
now im going to compare the results. but im not sure about accuracy of these results, so i want to validate theme by an valid article which has discussed about advantages and disadvantages of these loops.
for example whats the benefits of a PI-PI cascade control (which has been used in open pilot) in comparison with pixhawk attitude control loop? it is just an example.
this link (( introduced structures of the loops but there is not enough information about advantages and Disadvantages of those control methods in comparison with each other.
does any body know any source for this investigation??
thanks in advance