My research on the topic was almost 5 years ago, but I doubt much has changed.
I was given Kelty's top of the line carrier (older version of what you see in the REI link). It was pretty good for hiking, especially compared to inexpensive options out there, but was a bit uncomfortable on my back and shoulders. Also, the child rode a bit high up, causing me to feel less balanced as I walked. I sold it after trying out some others. The Kelty's are easy to find used on Craigslist.
The Tough Traveler Stallion is the one I still have and continue to use. The child rides lower, making my steps and balance more natural. It also makes overhanging objects less of a hazard. It feels more like a backpacking pack and can be converted to one with purchase of a conversion kit. It felt best on my back and shoulders. It is easy to keep the weight on my lower back/hips. It is lightweight, tough, and rated to 60 pounds. It ends up being pricey with accessories, but is made in the USA.
http://toughtraveler.com/comparecarriers.aspAlso pricey, but better than the Kelty for the same reasons, is the Deuter Kid Comfort III. I got it online on a great sale and sold it for my cost on Craigslist once I was sure I wanted to keep the Tough Traveler. It was good, but still made me feel more off balance and more uncomfortable in the back and shoulders than the Tough Traveler. I think it had a spot for a hydration bladder. The sun/rain hood zips into the pack when not in use.
http://www.deuter.com/en_US/backpacks.php?category=125 Others may have a different opinion on which is most comfortable, but it is hard to know what feels best without trying them out for awhile. The Deuter and Kelty should be available to try at REI (or buy and return if not satisfied). Tough Traveler can be returned within 30 days if you don't want it, and seems likely to give good repair service if needed.