This table shows the total annual cost for 4 Season Passes at Six Flags Magic Mountain California with the Premium Dining plan against 4 memberships during the Six Flags Magic Mountain CA Halloween Sale (September / October 2020).
This page compares the Six Flags Magic Mountain CA Halloween Sale 2020 costs for passes vs. memberships for 4 people. For a different number of people (1, 2, 3, etc.) see this page.
Season pass is a better value, saving anywhere from $15 to $435 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.