Massive essay on slings follow. Summary at the end.
I had Kari-Mes (very similar to Moby but not as pretty), several wraps, a ring sling, a mei tai, an onbuhimo, an Ergo and a Wilkinet. I basically didn't use a buggy after I got the Kari-Mes.
Wraps - stretchy such as a Kari Me for a newborn, but they start to get overly stretchy when DC is about 7 months or so, depending on weight. After that I went onto wovens - I have a GypsyMama Bali Baby Breeze (they've got a new name now though, not sure what it is), and a Storchenwiege wrap. Of all of them, I've had the best use out the GypsyMama - it's lightweight, folds up very small and is very very pretty. It's not very comfy for an all day carry of a stonking great toddler. That's what the Storchenwiege was for.
Ring Sling - less knowledge of these, didn't use them so much. They are great for when you want to look stylish and not crush clothes underneath (I got mine for a wedding) very good quick on and off. If you can get the knack they're very handy for BFing in. But many men feel they are girly, and they are a bit harder to wear a coat over because of the shape of them. (Of course you can wear them outside winter clothes but I preferred to put baby under coat so you don't have to faff with so many clothes)
Asian style carriers: Mei Tai, Onbuhimo and Ergo all variations on a theme. Wilkinet is kind of in this category too. Of all of these I found the Ergo type carriers by far the most comfy. I found the Ergo was easiest for managing a 6 month to 18 month baby on trains etc - very quick and easy to click child in and out without standing up. It's very DH friendly as it looks completely manly, and the pockets and loops very useful. They are easier to get child on your back than anything else I found. I found the best carrier for feeding in was a Yamo (very similar to an Ergo but the buckles can be adjusted both up and down so you can move their position on your armpits.) I found onbuhimos the absolute easiest to get a child on the back, but they aren't as comfy. Unlike onwardandupward, I found the Wilkinet a bit of a pain in the backside because you have to tie the child into it, making it very inconvenient for ups and downs and DH couldn't get on with it at all. I didn't find it nearly as supportive as a wrap, but my sister liked it.
If you want to sling exclusively you will probably want 2 or 3 over the course of your babywearing career. If I had to choose one, it would probably be the GypsyMama wrap as I got most use out of it and it was the most versatile. If I could have two, I would add the Ergo, and if I could have 3 I'd add in a stretchy wrap for a newborn.
Phew, sorry that was an essay