If I can stick my 2 cents in here

, while I do find using an sd card slower, your books will still be there in the event your Kobo dies.
I had installed 200 books on the reader memory. A day later I somehow I corrupted the firmware and had to do a reinstall of the firmware. All my books were deleted, luckily I had backed them up. After that, I would never go back to internal memory storage as I now have over 1300 ebooks.