LGW over LCY is rather obvious.. there are more time slots available as there is a limied amount of space at LCY unlike LGW and also the landing fee's are much cheaper at LGW than LCY. LGW with the rail connection I guess allows connections to various points in London for the business & lesiure passengers. Would also think that LGW allows connections worldwide with BA, VS, EK etc.
Regards,
Chris