There may not be, depending on your stats.
The Plate offers 3 more AC, and slightly more PRR (because of the way PRR is calculated you get less benefit from each subsequent point added). Medium armor, on the other hand, offers the potential for 5 more dodge, and a smaller armor check penalty to Balance, Jump and Swim. Again, these are small numbers, but when you're trying to squeeze every last potential benefit out of your choices, Medium armor may offer a slight edge.