Welcome aboard, Leslie! I'm excited to hear that you're about to set sail on the beautiful newly re-imagined Disney Magic.
While this is always subject to change, currently, the price to pre-purchase the Disney Cruise Photo CDs and the onboard price are the same. There are a couple advantages to both pre-purchasing and waiting until you are onboard. If you pre-purchase the unlimited cd of photos, you can board the ship knowing that you should take advantage of every single photo opportunity you see, as you'll get to keep each file at no additional cost. So, you can swing by to see Mickey, then Minnie, then pop in on Peter Pan and make a stop to duel with Captain Jack! Pose and smile to your heart's content! Especially if this is your very first cruise with Disney, I recommend pre-purchasing the cd so you know you'll get to keep all of those memories!
Alternately, as my family does now that we've sailed quite a few times, you might choose to wait and check out your pictures each day in
Shutters, the onboard photography shop. You will see prints as well as be able to browse the pictures at one of the digital photo kiosks. You may find that only, say, 10 of the photos are absolute must-have's for your family. In that case, you would be better off purchasing the 10 picture photo cd package. But, if you like and want them all, simply purchase the unlimited cd onboard.
Just remember to pick up your photo cd the last morning before you get off the ship. Unlike the PhotoPass system at the Walt Disney World Resort, guests with Disney Cruise Line are not able to view their photos or purchase photos online once they have disembarked, so be sure to pick out the best ones while you're onboard, Leslie!
Happy planning! Please let me know if I can help with anything else before you set sail!