We always rented prior to our purchase. If you scout out the rental yards, we found they had 3 "classes" of chippers. The "dinky" homeowner ones which really are slow and don't eat much, ie faster to just haul off and pay for disposal vs time spent chipping, then there is the smaller tracked chippers, which are versatile for "off road" type use and really do eat most everything except log wood over 5" inches, but the best machine we have found to rent was the Vermeer 600 size machine, it eats 90% of any branching, and if you feed your small sticks into it with larger stuff, it just eats it right up. We ended up purchasing a used Vermeer and use it as needed.
We also "sell" this service to the clients as, "Well, we can haul all this off for X, or we can rent the chipper for Y, and save you......" Usually sells itself at that point.
Again, you have to have enough material to chip for this to make sense.
Good Luck.