Closing Recommendation
Both machines come from the same maker one size apart, so pick by the shape of your working face and how often you move. Choose the SANY SY500C-10HD when your face is hard, high and tight and you need maximum bucket force to tear material loose, and when you shift sites often enough that the lower moving cost matters. Choose the SANY SY580C-10HD when your face is wide or deep, or your trucks stand further back, because its longer boom and arm let you cover more ground from one position and its wider bucket choice suits mixed material. Get a written service response time from the dealer either way. So match the machine to your work: SY500C-10HD for hard tight faces and easier moves, SY580C-10HD for wide, deep digging.