I know that when the creeps are mega they only give half the normal amount of gold. Is there a difference in the amount of experience gained by killing a normal creep vs a mega creep?
Yes they give less gold & experience.
When an enemy's Barracks falls, all Lane Creeps of the associated type for that team begin to spawn as Mega Creeps. Mega Creeps are stronger versions of regular Lane Creeps that give less Gold and Experience to the enemy when killed. When all enemy Barracks are destroyed, Upgraded Mega Creeps begin to spawn.
Updated of 7.07c
Ranged creeps give 69 XP and melees give 57 XP.
Mega ranged creeps give 22 XP and melees give 25 XP.