I have used Florida Tix and booked our tickets this year with ATD.
Previously I have bought tickets with an operator in order to qualify for free dining but we are staying off site this year so were not restricted.
Most important thing for me is having real tickets. Last year we bought 2 Universal tickets from Florida Tix and received physical tickets before we went and bought the other 2 Universal tickets from Virgin (I converted airmiles into 'The Virgin Voucher') The supplied us with paperwork that needed to be exchanged for a ticket at the gate. We had to queue for 30 minutes (well DD and I did) DH & DS were able to go straight in!
ATD also allow you to pay a 10% deposit - meaning you can secure your tickets at a good price and pay the balance 4 weeks before you go and still get physical tickets in your hand - makes them an all round winner for me!