I went back and forth between my maya wrap and something similar to the moby wrap. I LOVED my maya. Mine had an open tail so I could throw it over my girls as a shield from the sun/rain/snow or if I need privacy breast feeding.
My "moby" was just a long length of fabric I used. I had two. One was a light gauzier type fabric and the other one was a nice cotton; I picked both up at my local fabric store. It takes a bit of practice but it's very comfortable. I have a friend that could tie her son on her back using this type of wrap in what seemed like the blink of an eye. She said she'd practiced a lot in front of her mirror in the bedroom so if she messed up, the baby would fall on the bed. She also often practiced with a doll and it's what she recommends. I was never confident enough to do it as fast as she did but I would have been if I'd used this system more.
The mei tai's are pretty but they reall aren't as convenient to use, IMO.
Now, to clarify here, I have only recently stopped carrying my 4 yo, within the last 6 months. All of these systems could be used to carry her, but I still preferred my maya wrap or the moby (I especially loved this when K was really little, on the front) just for convenience and ease of movement. My family does own a van but I walk almost everywhere, rain or shine so I'm talking from a lot of experience here, not just as someone who might wear the baby once or twice a week. I practically LIVED in my maya wrap. Not only did I buy it used, but it withstood heavy use from two little girls AND I passed it on to my sister. While it doesn't look brand new, it's certainly still very sturdy. I have heard a lot of people say that their maya wrap hurt them but in my experience, it only ever hurt when I had it adjusted wrong. I also found that it's better to use a sling WITHOUT the padding. The padding prevents you from tightening it properly and adjusting it properly. Also, as with Over The Shoulder Babyholder, you had to buy a specific size; they weren't interchangeable. I had one when my oldest was a baby and absolutely hated it and my husband couldn't use it but he could use my maya wrap just fine. So if you want a more traditional style sling, maya is definitely the way to go.