I only played once as Austria and choose the decentralized path just for sake of roleplaying alone. Always fun to break countries as much as possible and then welcome their offsprings to the HRE club. As I understand the main difference should be the size of the armies. As non-vassals they dont have the limits and can field larger armies each on their own. Of course, you dont get vassal income either. Other benefits might be the casus belli (long before imperialism) itself that allows you to attack bigger neighbouring countries,, though partly situational