I also love my sling, but these days, if i carry dd ( 8 months and 25 pounds) for long in the sling my back goes out. With the hh, I can stand up straight instead of pushing my hip out a little like I do with the sling. the hh, I think, is actually pretty versatile. I can carry dd on my hip, or I can swivel her around to the front, or also around to the back. also, dp loooves it.
I still use the sling when I'm trying to get her to sleep, because I've never been able to get dd out of the hh without waking her up.