D.G.
Maya is a ring sling, and most of the weight is distributed across one shoulder. (I have heard if you wear it for long periods of time that you can switch shoulders).
The Moby wrap is a tie wrap- a long swatch of fabric that you can wear/tie in different positions. It distributes the weight more evenly. I have neck problems and the Moby wrap was hands down the most comfortable carrier for me, could wear it for hours. However it isn't as good for really hot days since it is kinda like wearing an extra t-shirt.
Both grow with baby (are adjustable) but both have a little bit of a learning curve. I know moms who love both.
http://www.peppermint.com/guide.html has a comparison of different baby carriers.