What are the advantages of using p6 in place of P3 to a Planner / Scheduler

main Advantage in P6 i feel is Activity id numbering , and every thing in just right click group and sort and all .
 

Main feature in P6
 
Resource loaded and usage profile, Group sort , Filter, Open multiple projects, Calliopes and expand . Modify Activity IDs etc
for more information refer P6 manual, there is topic known as  move P3 to P6  you will get to know more,
 
HTH