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AT89C52 is older design than AT89S52 and basically there is no advantages of one over the other. If you look into Atmel's device list there is no AT89C52 at all ( http://www.atmel.com/dyn/products/devices.asp?family_id=604 ).
There are small diferenies between them, however, and one of them is maximum frequency operation: 24MHz for C version and 33MHz for S version..