A saving piece of any pup-pack we've had is a handle on the pack. REI used to make a great pup-pack but sadly discountinued, however the handle saved me (and my dog) several times on the trails. 

Using a handle we could help him on ladders, steep descents and even tricky water crossings. I think a lot of pup-packs still have it and i would never choose one without it.