The idea of a discount if you purchased your season ticket early was to encourage fans to commit for the forthcoming season whilst the current season was still been played out. The membership scheme did away with all of that.Why would they discount up-front payments if they want to discourage up-front payments and have everyone of monthly direct debit though?
As of right now, they’re giving people the option of paying in full up-front, but they’re not making it cheaper for the obvious reason of pushing people to go on membership instead.
Personally I'd sooner pay up front because I don't like direct debits. But I'm old fashioned.

