I always suggest to people that, if possible, they should borrow a carrier they're interested in before making the purchase. Why? Good carriers are expensive, they have differing features, and not all carriers fit all body types the same way. The shop where I work just started carrying a new carrier by the Folks at Boba, called the BobaAir, and I borrowed it overnight so I could get out and use it a bit -- partly so I could do a "quick and dirty" review of it for some friends, and partly so I was better acquainted with it when helping customers choose the carrier that best suits their needs. As part of that, I did a side-by-side with some of the other carriers we sell:
See how differently they all look when on? Just for reference, I'm 5'8" and my little guy is 26mos old, between 27lbs-28lbs and in the 50th percentile for height.
With the exception of the Beco Gemini, all of these are rated to a maximum carrying weight of 44lbs-45lbs. With the exception of the BobaAir, all are also designed to be used with newborns (though the Ergo carriers accomplish this with an insert that must be purchased separately). What isn't visible in the picture is how different they all feel on my back.
The BobaAir (what I would consider an occasional or back-up carrier) pulled down on my shoulders and put no pressure on my hips at all. It was more comfortable than I expected, though the lack of breathability in the fabric would make it uncomfortable for more than short-term use on a hot day.
The Ergo Organic felt like it was going to slide off my shoulders and the low rise made it feel as though my little guy was leaning too far back, putting me a bit off-balance.
The Ergo performance straps dug into my narrow shoulders and were itchy against my skin -- as with the Organic, I didn't feel as though my toddler was entirely secure (though I know he was).
The Manduca, my carrier of preference, put all the weight on my hips and, despite looking as though he was leaning out, felt a lot more secure over all -- I've used many SSCs and this is the one I keep coming back to, as it is comfortable enough I can still wear him on my back at work for a 2+ hour nap, if needed. It has more points of adjustment than any of the other five carriers pictured above.
The Gemini looks comfy on the picture, but the extra height didn't provide any more support than the height on the BobaAir and the Manduca. It also pulled on my mid-back in an uncomfortable manner.
Until you take a carrier out and walk around doing your regular activities, you really can't know what will be the best fit for you. Check the baby stores near you and see if they loan out samples of the carriers they sell -- many do for free (or a minimal charge). It's a great way to make sure you're getting the most bang for your babywearing bucks.