On the first day of training they will offer to the whole class base and equipment options. I don't believe MIA has been offered in training, it's mostly ORD/LGA 145, DFW/ORD 175 and ORD CRJ.
People pick in the following order: Pipeline cadets, pilots with enough prior 121 time to upgrade, and finally everyone else. I believe within each group it goes by age, but I could be wrong.