The only thing I would be worried about with attractiontix is that their website says you have to exchange a voucher. This can be a nightmare at Universal. In 2013 we booked with Expedia and I wait well over an hour, probably 90 minutes in the queue at guest services in August heat and with an very angry crowd. I would never ever get a voucher again. Last year we had real tickets but saw the queue for guest services and even longer. They really do have a problem with ticket collection there. Anyway, I would gladly pay another £10 each for a real ticket.
Personally I am hoping ATD do the free
legoland ticket deal again.. They have done it two years running and it was a great deal. Free 14 day legoland ticket. Would never have gone otherwise but we really loved it there.