I think he's chipping, not grinding. A Craigslist ad will usually find someone willing to sacrifice a Saturday hauling them off to save a couple hundred bucks at a landscape supply store. Failing that, most landfills have provisions for green waste.
If you don't have a use for them on your property, you will probably find out that once you've rented and picked up the machine, figured out how to use it, realized you stacked your brush all wrong, finally got the machine cleaned, fueled and returned, answered 10 replies to your ad that never showed, and finally hauled the chips to the landfill yourself...
That the bid you got for chipping and hauloff by Joe's tree service was pretty darn reasonable. Ive never seen a homeowner beat a bid doing it himself.