I think slings are great for newborns till 3 or 4 mths old. After that, baby's weight places quite a strain on the back. So I switched to baby carrier after that.
Having said that, I think it also depends on the baby. My 2nd one was superhappy in the MIM sling and he adapted well to the carrier. (My 1st one I didnt carry often, only hubby did when we went out. Told hubby that I alrdy carried him for 9 mths, so he muz do his bit of carrying.) Now our 3rd (he's 2 mths old) does not really like the sling. He's quite a fussy baby who likes to be carried. So I'm thinking of getting a carrier that's suitable for younger infants cos I think the ones we have are more for older babies.