Each airline is different and often depends on either its coorporate policy or conditions of the time.
For example, British Airways at one point always hired permanent crew (with an initial 6 month probationary period just incase you turn out to be rubbish) but this year offered 6 month temporary contracts to LHR longhaul. HOWEVER...These "temps" were then offered a permanent contract after 6 months down in the LGW mixed fleet. So in effect, they were offered permanency after only 6 months.
Suppose its all really down to timing.