Depends on soil type. In heavy soils a tiller may do better, but is much slower. Only disks that are probably too heavy for your equipment will really dig heavy clay. In loam or sand I choose a disk as it only take a couple quick passes to really turn the soil deep. Problems with a tiller, though, is that it has a bad habit of clogging up in slightly moist clay soil types. I use both depending on soil type and moisture content.