Whether you purchase a Membership or Pay by Area, 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 6month or 1year 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 areas 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 an area, it's accessible to all your characters on that account. Purchasing individual areas lets you progress at your own pace without feeling like you have a Membership hanging over your head.
The prices for the individual areas 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.