It depends on how many days you are going to stay at a certain park system.
because we go twice a year, we buy Annual passes for both WDW and Universal, as after around 10 days they pay for themselves. Both offer discounts on renewal as well. In fact with Universal they just "reload" your plastic photo pass into their computer and its ready for your first visit. WDW its new passes every time.
These you can only buy either online direct from the park company or at the gates not via a thrid party.