I asked the same question and was told basically the same as listed in David's post. The exception is that when mail is loaded on the plane Priority goes on first [so they say] if there is an abundance going to the same location some of the first class may be left behind for the next flight. In the rare case where weather may become a factor Priority would get there first.....
If you are sending a letter my opinion is if you are in a rush... Send it Express otherwise .33 is just fine...