Depends a bit on the dog, I think.
Necks are handy because they're so incredibly easy for the dog to eat. One crunch (often not even that) and they're swallowed. They can thus be a good part for young puppies (for whom they're relatively large) or for tempting reluctant eaters. On the other hand, they're not a good thing for gulpers. If your dog is inclined to inhale his food, then they're a choking hazard.
Backs are altogether a great deal more work - for the dog. It is impossible to swallow those whole, so the dog spends a fair amount of time chewing. That's great for his teeth (and for recreation - he's most unlikely to go chew your shoe afterward

), and for overenthusiastic eaters/gulpers. But a reluctant eater may decide it's too much effort and give up half way.
So the answer to which one is "best" depends a little bit on your dog, and on what else you feed. A mix of both can satisfy most requirements though (but it's wise to feed the backs first, necks after for all but the most moderate of dogs).