All of the models I used have been updated since I used them, so I can't recommend one over another per se. I used an old style (90's vintage) Tough Traveler which I never could make comfortable. I sold that and got an REI Piggy Back, which I used until my son was almost 4. I then went with the Madden Caravan, which I think is better for longer/older kids. I LOVED the design of the REI pack for it's ease in getting kids in and out, but it was never quite comfortable. Tolerable, but not particularly enjoyable. I'd heard lots of great things about the carrying comfort of the Caravan, but was sorely (literally) disapointed when I finally broke down and bought it (sight unseen since nobody in AK carries them). I tried the Kelty in the store, but my son detested it, and I didn't think it was all that comfortable on me. I had a friend who returned 3 Kelty packs (suspension adjustment kept slipping) before getting the REI pack and loving it (they were definitely power users). I had another friend who had a Kelty for her first pack and happily upgraded to the REI Piggyback with her second kid.

I think the main thing to consider is comfort for the carrier first, and safety for the child as equally important. I think all major packs have been upgraded to 5-point harness systems for kids... I'd avoid any pack that doesn't. And then, based on my experience with the Caravan, I'd say select the most comfortable pack you can try on and "test-drive" with your child before you buy.

MNS