I would be interested to know what importance passengers really put on there being a good "shopping experience" when choosing a departure airport.
Personally I would rather choose somewhere which offers a speedy transit through the terminal so that I don't feel I need to arrive 90 minutes before departure just in case there are delays at security, or I have to wait 20 minutes for the car park bus. At SEN that speed of transit is available but when I use LTN or STN even I (and ask my wife how much I like shopping!) do find it relieves the boredom to browse in the numerous shops. But whether most other people appreciate the shops simply for that reason or see them as an essential part of the travel experience I really have no idea. I suspect research must have been carried out into that subject in the past.
SEN certainly offers sufficient retail to meet the basic needs of the traveller as Aero Mad says, but what I really appreciate there as opposed to LTN is that you can actually sit comfortably adjacent to the gates until your flight is called. Standing around for ages at the gate, or even worse on the stairs at LTN, is no fun in my book.