I have been/am a carrier and sling addict. I wrote a review of all of the ones I had at that time on my blog.
http://samanthakawakami.blogspot.com/2008/07/baby-slingca...
Since I wrote this blog, I have one more to add. The Ergobaby. This one is my absolute favorite. If you're only going to buy one, I recommend this one. For infants, you need an insert. I've never tried this because I bought the Ergobaby when my youngest was 10 months. But my sister used the Ergobaby with her infant and loved it.
Key points of the Ergobaby carrier:
1. The weight is all on your hips, so you don't feel it! You do not get the aching shoulders and headache that you get carrying a heavier baby/child in other carriers and slings. (The first time we used it hiking for a whole day is the first time that I carried our daughter for the entire day without wanting to give her to my husband after an hour or two.)
2. You can use it as a front, back and hip carrier. I've never tried it as a hip carrier. But being able to use it as a front and back carrier is fantastic. It gives you a lot of flexibility.
3. It's a bit tricky to get your child on your back until you get used to it, but it's very secure.
4. In the "pouch" where the child sits the fabric between their legs is wider (especially compared to a Baby Bjorn carrier). There are two advantages to this. One, it is better for the baby's hip positioning. Two, it means when they're just a bit bigger their legs are out to the sides more and not positioned right above your legs, making it difficult to go up steps because their legs are on top of yours.
I still use the Ergobaby with my 21 month old. She loves it.
I do still have two slings (a Maya Wrap and a Zolowear) that I plan to use with my upcoming baby. (And I sometimes use these to snuggle my daughter too.) I like the slings with smaller infants because I think they are easier for nursing in public. You can slip the baby over in the sling and nurse them and no one can tell. (I've done this in a business meeting before!)
I also have to say that I am debating picking up another Baby Bjorn carrier at a consignment shop because I do like this one for smaller infants. I like the fact that they're very securely against you. And I also think that it's a quite a bit easier than the Ergobaby with insert. The only thing is that by the time your baby is 6 months, you definitely will not want to use this carrier with them!
Another thing to consider is that each baby likes to be held differently. My first one liked to be horizontal in the sling, but my second one hated being horizontal and wanted to be vertical. Slings do provide a bit more flexibility this way. And if you're debating between a ring sling and a hotsling, I would go with a ring sling. Hotslings don't have nearly the flexibility. In a ring sling you can adjust how the baby sits in the sling by tightening the whole sling, or just the top rail or bottom rail. I gave a Maya Wrap to my sister-in-law and she loved it, and lended it to her sister to use.
There are so many choices out there. Good luck with finding one that fits for you.