LCY is a better overall experience however SEN caters perfectly for the SEN catchment which is those people living in the 20 miles to the West and North of Southend.
Two things will dictate how well Southend does; flight schedules and flight prices.
People soon forget queues, bad customer service or lack of facilities if the destination they need to get to is served by aircraft leaving and returning at convenient times and at a price they like.
Which surveys about customer satisfaction mean nothing - just compare Luton and the customer experience there to the passenger numbers they attract. The management at SEN need to attract airlines that pay their way and not rely upon their own airline to fill stands without profit.