Hi, Laura!
If you are going to want to order several of the photos taken by Disney's professional photographers, then you should seriously consider taking advantage of the
Disney PhotoPass + preorder discount. By ordering this through discount link you'll save $50 off the regular price, bringing the cost down to $149.
We like to commemorate the family trip at the various stages with photos in front of Walt Disney World icons such as Cinderella Castle or the Tree of Life. We'll cherish those photos in years to come, plus they are of a far superior quality than I could take with my camera, AND we are all in the shot. IF you plan to pose for several of these, as well as, maybe, photos at select attraction and Character Greetings, too, then this plan is a good way for your to save money while getting those memories captured for future enjoyment. On the other hand, if you are not going to want to stop for such photos, then there is no need for you order PhotoPass +.
The key here, is to really think through if you'll wish you had these photos upon your return. If your answer is yes, then the preorder is the way to go. If you'll use it, it's worth it. If, however, you think it will be too much trouble to worry about posing for photos, then skip it this time.
Hope this helps you choose! Send more questions as you think of them, Laura. I love to help!