It's because both of the RQ versions include a special hardcover art book. This is in addition to the artbook that's already packaged with the game, so the $10 covers a second, much larger artbook. I deleted the e-mail, but they said it was the biggest artbook they'd made thus far.
The PS2 version is also $10 more mainly because of the second disc. The second disc includes Japanese voice overs, whereas the Wii only has English. Amazon has the MSRP listed incorrectly for the Wii version. It's supposed to be $30 for just the normal version.
Edit: Found the e-mail, here's the text:
No joke! RosenQueen has made a special preorder item for Sakura Wars. You can get an awesome 40-page hardcover artbook, "Unforgettable Memories," with your preorder! This special art book will be available for both Wii and PlayStation 2 versions. The PlayStation 2 premium package of Sakura Wars will already come with a Japanese-voice disc, an art book and a poster as bonus items, but this RosenQueen exclusive hardcover art book is different from the one in the premium box! So if you decide to go with the PlayStation 2 version, you can enjoy two different art books offered by NIS America and RosenQueen!
This RosenQueen exclusive art book is 8" x 10" in size, which is the biggest art book RosenQueen has ever made! With this art book, you can fully enjoy all the character art from Sakura Wars for sure!