If you're going to use it for photography then you may as well get the 64GB version. You won't be upset if you have a bit too much room but if you run out..........
I have just returned from Norway where i used my iPad as my primary storage device. I have 30-35GB left on my 64GB iPad after programs, books, movies, magazines etc. Every night or so I uploaded my photos onto two devices (iPad and portable hard drive). Then I formatted and reused the cards. I culled the pictures on the iPad but not on the larger portable drive. So I ended up with two copies of the "keepers" and at least one of the rest. I shot just over 3.5K pictures in 5 weeks. About 1K are still on the iPad now. The IPS panel in the iPad was great for viewing and culling. I used Snapseed and Photosmith to sort and rate images for when I got home as well as uploading a few to Flickr. Most nights I would loose the iPad. Usually I would "find" it in my wife's hands, surrounded by a large group going over and critiquing the days shoot. The easiest way to find a missing iPad is to follow the sound of the "oooh's" and "ahhh's".
As an aside, while we were away my 9 year old took 4 books to read on the trip. She finished three of those on the plane. It was a real benefit to be able to buy and download the next Molly Moon book in the series for her so she could continue her newfound love of books. In the end, the iPad meant she was able to finish an additional 4 books while away.
Gordon