I could write a pages long biomechanical and technical thesis about these two kicks (actualy about every kick

), but because of a lag of time I just sum it up and leave the exact explanations aside. A back kick is faster, stronger and more stable but also more "moveable" than the spinning side. The spining side kick is actualy technicaly more dificult but alows higher precision.
Back kick requiers a body turn of 90-160° (depends on the stance)
Spinning side requires a body turn of 160-240° (depends on the stance)
SSK=longer turn=losing time
Before the leg reaches out:
Back kick: knee is directed downwards
SSK: knee is directed sidewards
It gives: the center of gravity is higher and more excentric on the SSK=less stable
Body can be bend down more on the back kick which alows to lower the center of gravity (stability) this also brings the center of gravity closer to the force vector-line (stability power).
etc. etc. etc. I could go on and on, but I think you catch my mind, back kick is better.