I do think in many cases the need for a medium and heavy duty cutter is over estimated. Once cleared even thick brush can be handled with a light duty mower if you cut it once or twice a year. And after that, all the money you paid for the higher 'duty' level is a bit of a waste and the extra weight is just, well, extra weight.
Obviously, if you intend to cut 'brush' all the time, then the need is there.
My LX-6 is labled as light duty by JD. However, its specs are close to some so-called medium duty mowers and yet weighs only about 650#. It has 10ga decking. Blades are 1/2 inch thick and 3" wide (most mediums are probably 4"). The gear box is rated to 60hp. Driveline is Cat 3 and I have no idea what that means but the mediums are rated higher and I suspect this is what makes the LX-6 light duty. It does have a slip clutch. It is only rated at 1" brush. I'll cut 2" stuff without hesitation but do not cut 3" stuff. Three inch brush looks huge to me.
I've had it for three years and it works fine. I've hit lots of rocks, stumps and fire ant hills. I also mowed this all day long:
And it looked like this afterward, no problems:
Bottom line, no need to spend extra money for specs you won't need. I paid 800 for my LX-6, used.