IMO there are a few things to consider when buying a sling/baby carrier:
1. Most important is safety. Is the baby's chin touching their chest (like in the Sling Rider)? That will cut off air flow and can lead to suffocation. Like a pp mentioned you should always be able to kiss baby's head or it's just not safe. A baby recently died in a ring sling as well. Not because the sling was defective but because the parent curled them up in a "C" position and the baby suffocated.
Also, part of the safety issue is having a carrier that does not harm baby's developing spine or hips. Carriers like the Bjorn where baby faces out basically suspend the baby in an unnatural position from their crotch. Picture being suspended from your underwear for a while. Not comfy. Carriers like that can pinch nerves and cause problems in developing spines and hip joints. Basically, stay away from anything that faces baby forward with their legs dangling out.
2. Longevity. You want to get your money's worth. So you want to be able to wear the carrier in different ways, wear it comfortably for long periods of time, and as baby gets older and heavier. Many good carriers are very versatile and can be used from infancy-45lbs. The Bjorn says it can be worn to 33 lbs, but the way it's designed. . .it would not be comfy to wear a baby over about 20 lbs. Also with a good carrier as baby gets older you can do a back carry.
3. Comfort-YOU want to be comfortable right? Consider that as well. A more versatile carrier (such as a soft structured like the Ergo or a Mei Tai) can be worn in front or back. Again, don't get a carrier that dangles baby from the crotch for their comfort. And a carrier that has the option of a back carry will be much more comfy as the baby gets heavier.
4. Price. You can do some research and find out what kinds of carriers you want (I suggest trying on anything and everything possible) and buy them used on the baby wearer.com or diaperswappers.com Babycenter even has a baby wearing swap. You get a great carrier and save some $$
