Whether you purchase a Membership or Pay by Zone, really depends on how you think you will play the game.
If you plan to push through as much of the content as possible in the shortest amount of time, then a month to month Membership is the best value for you.
If you plan to play several characters over a long period of time, then a 6-month or 1-year Membership is probably the best value for you.
If you only have a few hours a week to spare, or will hold off playing for long periods of time, then purchasing individual zones is the best value for your time. It allows you to purchase just the next street you need to progress, and once you've purchased a zone, it's accessible to all your characters on that account. Purchasing individual zones lets you progress at your own pace without feeling like you have a Membership hanging over your head.
The prices for the individual zones can be found here: https://www.wizard101.com/game/areapricing If you're not sure, then use the Free to Play portion of the game as your guide. See how fast you move through to the end of Triton Avenue, and gauge your passion and your available time accordingly.
During the summer I buy a $20 2 month subscription pre paid card at Best Buy because I tend to play 3-4 hours a day (whoops). For the rest of the year I pay by crowns because gotta maintain that 4.0 GPA ;)