Buying a stroller will certainly work out cheaper and has the advantage of being able to wheel the tired kids all the way to the car.
That said we always rented a stroller
The reason being, last time we used the strollers in the parks the kids were 7 and too big for the cheap Walmart strollers,the park strollers are sturdy and give good protection from the sun. Also, we often just left strollers and picked up a new one from a different part of the park - if it was our own stroller we would need to make sure we returned to get the stroller.