How do they compare? Does the AFD focus any faster or more accurately than the AF? I've got an AF and the focus is slow and misbehaves frequently. If I really want to get a fast-changing shot, I'd better be focusing manually. When it comes time to get a second body, is there any benefit to getting a more pricey AFD (original version) over another AF? I'm have no plans to shoot with digital backs so I don't need that capability. Thanks!