Holding a convension first is a disadvantage:
Democratic National Convention July 26 - July 29
Republican National Convention August 30 - September 2
Democratic National Convention August 25 - August 28
Republican National Convention September 1 - September 4
Holding a convention second allows for, among other things, picking a Vice Presidential candidate that can effectively parry that selected at the first convention. It also tends to replace the story of the first convention, and that "message" can last several weeks - which is politically useful late-breaking momentum.