Both the Disc Harrow and the PTO powered Roto-tiller are different forms of soil mixing tillers.
Disc Harrows have to be pretty heavy in order to cut well. Generally speaking 18" diameter pans are the minimum, therefore around a 30 horsepower, 4-WD tractor is required to pull a Disc Harrow with 18" diameter pans at the brisk pace necessary to mix soil. A Disc Harrow is a secondary tillage implement. Usually new ground must be "opened" with a plow before a Disc Harrow is effective. Once plowed, a Disc Harrow can maintain tillage if used once or twice per year. Disc Harrows are most often used in fields.
(Quite a number of variables to be considered.)
Roto-Tillers mix soil well on even 20 horsepower tractors. Roto-tillers are preferred for garden situations, where short pulls, rather than long pulls, are the norm. With repeated coverage of the same ground, may prepare unplowed ground. Best paired with an HST tractor transmission because forward movement will be slow while the implement churns the dirt. After being prepared with a Roto-tiller most soil is seed ready. For sod preparation, use a Roto-tiller.
In summary: small area, small tractor = Roto-tiller. Field work, larger tractor, heavier implement = Box Frame Disc Harrow. Both are soil mixers.
With either implement be cautious not to overwork the clay or it will become adobe brick when dry. Work in as much organic matter, preferably composted organic matter, as possible.
Removing rocks usually requires stoop labor to deposit rocks in the FEL bucket or a trailer.
