Biggest thing that bugs me about the normal season passes is that no matter when you buy it in the year, it expires at the end of the year. Often it does get cheaper later in the year, but you're still paying for only part of a year.
The Membership is the same price annually (at least as compared to buying earlier in the year, like right now), but is good for the full year.
It also helps that Magic Mountain is open year round (just often only on weekends).
The one downside is that you do indeed need to take action to cancel, instead of action to renew.
I've always gone with a Season Pass over a Membership and will continue to do so as it is not that expensive and my visits to my local Six Flags (Magic Mountain) tend to vary in frequency. Some years, I've gone six or seven times, plus visited Six Flags parks in other areas of the country, while in other years I've only gone once or twice. As a general rule, if I don't anticipate four Six Flags visits in a year (total, not just SFMM) I don't buy a pass and just pay per visit (with discounts, $30-40 per visit is not uncommon).