I would say LCY has much higher earning potential. Why? Because the airport is in the business end of the market so aviation fees and charges will be high which can be partly off-set/absorbed by higher airfares.
SEN cannot be close to GBP2bn resting on the back of a Ryanair base which could (and will) pull the plug at ANY moment if fees/charges don't remain FREE or as close to FREE as possible. If SEN don't have land to earn money from car-parking then I can't see how they can sustain such low fees/charges for airlines using the airport.