After you clear the rubbish, what role will the cutter play? If it's hammering down 4 inch stuff for its life, then I'd look to the heaviest gearbox and driveline... if its going to be maintaining nice fields, and you want a nice cut, higher blade tip speed makes a difference there.
You can throttle down a high ratio gearbox, but you can't speed up a low one.
The dedicated heavy (3.5"-4")shredders do not seem to turn up as high as the medium duty ones (2"-3") do, but given their roles I can see why... there are probably exceptions.
The most complementary support I have heard has been from Bush Hog, and the Alamo products (Rhino, Alamo--others?). I haven't heard a complaint about Brown, very few about Woods and Land Pride.
If I were buying new again, I'd check into driveline repair/replacement cost for my finalists.
Bush Hog components are readily available all over the web... some others assure their set-up is proprietary and costly (my JD for instance--replacement input gear and output shaft only were $100 more than a whole new (non-JD) gearbox and driveline with slip clutch!), and aftermarket support will be non-existent.
It's nice to be able to do a repair for hundreds, rather than thousands of dollars...