Help! Ultra 6 or Pro 6

Thanks - the warranty is nice but for £190 doesn;t get me there - had faultless service from the NV. On backups I don't really want two NAS's running and not sure I really need a redundant backup. At the moment for all my photos and documents (the only critical items) I have the original working copy, backed up nightly to a second drive internal to the Mac Pro, and then backed up nightly onto the Readynas. So would need to have 3 concurrent failures before loss? I am going to augment this by rotating a weekly backup on a USB portable drive that I will store at work. Does that sound reasonable or am I missing soemthing in using the existing NAS?