Jet2 738
It all depends on your market segment, i haven't seen Jet2 passenger numbers for 2014 but guess around 6m, easy jet are of course in a different league and to be honest market place.
The vast majority of people of people using low cost airlines are doing DIY holidays and with 90%+ of Booking.coms hotels you pay nothing until you turn up or have the option of cancelling 48 hours before, my personal observation of people using Jet2holidays.com is that of chasing a bargain and in most case could have put the same deal together other than transfers for less, yes Jet2 are ATOL protected, but you are a 100% safer in terms of the commercial risk with easyJet or Ryanair than Jet2 anyway.
The one advantage of booking a Jet2 holiday ( or any IT holiday) is that they have cradle to grave responsibility to sort it out when things go wrong, but in the South of England at least its a dying market for European traffic and Jet2 don't have a long haul product, even the last bastion of all inclusive the ski market is giving way to DIY, finally don't forget ATOL comes with a cost to the airline and the punter.
Book your flight well ahead and book your accommodation with any of the major online providers and you'll still pay less up front, plus you choose the level of accommodation you want, there is after all a reason why you can do MAN-TFS-MAN all inclusive with Jet2 for under £250 a head, a lot are crap hotels, but its the same with any IT airline