You're in the great land of compromise, Jeff.
Small and portable often times does not equate to high production numbers on primary grinds. I have a small grinder (Sorry, its a tub...I know, you're looking for a horizontal grinder), and unless the job is immense, its often quicker to load out our 40 yard cans packed full than it is to grind up and load out. Even if it is huge, I send the tub to grind some while the truck is out, and the cans still get packed with unground materials. We mulch everything at our yard when the pile is huge and the sky is crying.
The folks I know that have a larger grinder all "need" a walking floor and tractor to keep up with the output. Unfortunately, the solution to their problem was to buy a $100,000 machine that required a $50,000 trailer that takes 10k in maintenance a year, and a tractor to boot.
As far as the maintenance, I won't pussyfoot around this....you'd be buying a machine that karate chops wood at 5000 rpm. I don't care if its a tub or a rotary grinder, if it smashes stuff all day, stuff is going to wear out.
I'm all for the right equipment for the job, but our best bang for the buck was not to reduce volume, but to increase hauling capacity. We bought a used Western Star triaxle rolloff and dumpsters. If we were running a truck anyway, might as well make it 40 yards instead of 20. It wasn't a 3:1 reduction that a grinder could promise, but 2:1 reduction in transportation costs was pretty good for the 20k we paid.