most of the over-the-shoulder slings are very comfortable but get old when your kid gets heavier. I like the Kangaroo Korner adjustable fleece pouches and have heard that the maya wraps are nice too. Really, any wrap type sling (like a Storchenwiege or Didymos) can be tied to carry a hip carry too.
Since your child is older and bigger, you might also consider doing an Ergo baby carrier. You can carry this on your hip and because it has a waist/hip component in addition to the shoulder straps, it puts more of the weight on your hips. So it's more comfortable for long carries. Incidentally, you can tie a wrap-type sling to do this too. I suggest heading over to Be Bye Baby or a similar store that sells several types and trying them on with your baby. You can also look at reviews at www.thebabywearer.com