Amy,
You definitely can wait until you return from your trip to make that decision. However, the delay will cost you.
You can pre-purchase a PhotoPass CD at a nice 20% discount if you use this link:
PhotoPass Pre-Order That makes the current (9/08) price $99.95 - regular price is $129.95. When you return from your vacation obviously the pre-purchase offer is no longer valid, so that would cost you an additional $30.
If you opt to order individual pictures, expect to pay $12.95 or more per picture. That means that, for the same price as the pre-ordered CD with all of your vacation photos on it, you could order prints of only 7 photos. For reference, we just returned from a Disney trip a couple of weeks ago, and we have over 300 pictures on our PhotoPass CD. That does not include any pictures that we took ourselves, which can also be uploaded and put on the CD.
I think the PhotoPass CD is worth every penny, and something that we buy every trip. I highly recommend it!