Hi,
Not sure what MBP is? Since being in the military equals being outside the US so much, I would stay away from the Barnes & Noble readers. They only support readers that are in the US. It doesn't matter that you have an APO address or that you are American, they won't even let you update the firmware from abroad.
Since you mention the military connection, I would steer you toward the 5" models, just because they are so portable. I can tell you from experience that the Sony PRS 350 and the Astak PocketPro both fit well in both the cargo pocket and the pants pocket of the ACUs. Astak has stopped distributing the PocketPro, but they are still being made by Jinke (Hanlin) and sold by other distributors. The PocketBook 360 has a nice cover that snaps over the screen to protect it, but I thought it was a little wide and heavy for the cargo pocket and definitely doesn't fit in the pants pocket.
I'm also not sure what you mean by open source? Do you mean not tied to a book store? Something that can be easily rooted or uses alternate firmware? As far as I know, all of them allow you to attach them to Linux, Windows, or Apple computers. Generally, the reader will show up as a USB device. The proprietary part comes in with buying books. Electronic books from Amazon only work on the Kindle. Ebooks from B&N only work on the Nook and Ebooks from iTunes only work on Apple products. There are also lots of other book stores. Some, like BooksOnBoard and Borders, sell mainstream best sellers that will work on readers with Adobe Digital Editions. Others, like Baen (science fiction) specialize in self-published or genre. If you go to
www.inkmesh.com, you can type in some titles or authors and see where they're available.
Because of geographic restrictions, some US based books are either not available to you in Germany or cost more. A VPN that gives you a US IP address, combined with a US based credit card, will let you purchase from most of the book stores. It's also technically possible to strip the DRM from most books and then convert them to work on your reader. We can't go into specifics on that on MobileRead, but Google for Apprentice Alf for more details.
It's very much personal preference, but most important to me are very good display, ergonomics of holding the reader/turning the pages, and organization of books on the reader. I like to carry a lot of books at one time; on some readers, the only organization is a list that is alphabetical by title or author. I've never missed WiFi, because Calibre does such a great job of putting the book with its metadata on my reader. The Sony 350 meets my most important requirements and travels well, so it has become my favorite reader. It has the advantage of being carried by AAFES, so it should be available for hands on evaluation at the PowerZone.