As long as you're a SoCal resident, you can do the monthly payment plan. NorCal residents are NOT eligible for the payment plan option.
As noted, make sure you upgrade to the pass after you've used up any of the freebies that come with the tickets (MM, etc.). Keep in mind that once you upgrade, you get all the perks(parking if applicable, food discounts, merchandise if you have a premium), but those discounts are not retroactive.
Your best bet is to figure out whether or not the ticket perks (MM)are worth more than the AP perks(discounts) and upgrade accordingly. If staying onsite at one of the three DLR hotels, you get MM for every morning of your stay that it's offered.