They serve different purposes. A season ticket is tied to a specific set of fixtures — it gives a fan entry to a defined number of games at a fixed upfront price.
A membership is a recurring subscription that delivers ongoing benefits (like ticket discounts and exclusive content).
They can run alongside each other. Many clubs use both: a season ticket for fans who want to commit to attending every home game, and a membership for fans who want to support the club and get value alongside a season ticket or without a fixture-based commitment.
Some clubs may include a season ticket with their membership to roll up season tickets and additional benefits into one package.