Hello Lindsee and welcome to the Disney Parks Moms Panel!
What a great question! The two options can be a bit confusing. Let me see if I can help.
With Disney's PhotoPass Service you simply find a Disney PhotoPass photographer and join your family in the picture! Then ask the photographer for a Disney PhotoPass card and you can then use it for the rest of your vacation. At the end of your vacation (or, even during your vacation) you can log onto the Disney PhotoPass site, add your card number(s) and see all your photos online. You can then choose and purchase any (or none) of the photos you like.
Memory Maker in my opinion, improves on PhotoPass. With Memory Maker, you get digital downloads of ALL your photos at the end of your vacation. You can also add photos from select attractions and character dining spots to your Memory Maker. In fact, sometimes, when I log on after a great day in the parks, I am surprised by what appears in my account! Once you're home, you'll then be able to download all the digital files and have them printed in any way you prefer. There's no limit to the number of photos or the number of times you can download them. Make sure you order your Memory Maker at least three days before your need to use it- that way you get a discount! Another plus is the videos that capture you and your family on some of the attractions. My favorite video was captured on The Twilight Zone Tower of Terror.
My family has used both PhotoPass and Memory maker, and for us Memory Maker is definitely the way to go.
See ya real soon!