Because augers don't like wet, sticky clay or big rocks. When the digging gets tough, the tough get excavators. Doesn't sound like you will have a hard time with the digging. Maybe and maybe not. A couple old pieces of plywood to lay down under the tracks for turns will make a mini-excavator real turf friendly. Excavators just pack so much more digging power into the same size machine. Hyd. Thumbs are more common on excavators than backhoes too, which makes plucking rocks out of a hole so much easier.