The 3x20 series is deceptive, because it has such fantastic lift capacity & hydraulic flow rate... capabilities on a 2900# machine that would suit a 4000# unit.
It seems like 90%+ are either a lighter 6 footer or a medium duty 5 footer; either covers the tracks. With the loader on & loaded tires the are different machines that dry/no loader. Biggest cutter I have seen people using are the medium duty 6 footers, but get up over 1000# and theres a lot of leverage picking up the front when its raised.
I would look to Woods, bush hog, brown, rhino... I have an MX5 with the flawed output shaft & JD's bolt-on fix is a $2200 one, and they've broken at a rate that would have an automobile manufacturers line pulled off the highway. Based on this experience, I could never recommend their product... shortly after the initial purchase, before the breaking started everywhere, I still saw the listed manufacturers were more robust, had higher brush diameter ratings, a gauge up on the deck, and in Woods case, exceeded the blade tip speed on the mx lineup. I had buyers regret from not doing more research on the options, and the 0% for the cutter with the purchase made me an easy sale.
The other medium duty cutters are 10-25% heavier, so push the limits, not of the hydraulics, but frame capacity... still people run them successfully, just have to keep the front down/heavy.
Figure out what your application is, then go up a size!