I just spoke with a postal employee (my wife). Everything priority is done first. When a clerk is given a basket of mail to sort, priority mail is pulled first and put into its bin first.

Priority mail is given preferential treatment all the way down the line. Bottom line is that priority, for the most part, gets to its destination in 2 days. First class MAY get there in two days but may, and often does, take 3 to 4 days.

A side benefit besides speed is that there is a significantly less chance of anything happening to a priority package.