This table shows the total annual cost for 1 Season Pass at Six Flags Fiesta Texas with the Premium Dining plan against 1 membership with Premium Dining during the Halloween sale (September / October 2020).
This page compares the Six Flags Fiesta Texas Halloween Sale 2020 costs for passes vs. memberships for 1 person. For a different number of people (2, 3,4, etc.) see this page.
Season pass is a better value, saving anywhere from $18 to $130 over the year, depending upon the Membership level chosen.
Costs below do not include taxes. And Six Flags will probably slap on a "handling" or "convenience" charge when you go to check out, but those won't affect the analysis of which option is a better value as they apply across the board.