If you specifically want a carrier, rather than a wrap, look for any of the ergonomic brands, including Ergobaby, Manduca, Boba. Avoid Baby Bjorn or anything where baby is positioned with legs dangling down rather than supported in a sitting like position. They're bad for their hips and they'll also kill your back eventually. You'll also want it to be something where baby facing you (inward, rather than outward). Personally, I would recommend trying a stretchy wrap like a Moby wrap with a new baby. They are so much more comfortable than a carrier, especially for doing things like bending over, sitting down, getting down on the floor, etc. which you will do a lot with a toddler as well.
A carrier will cost more and they aren't as comfortable for wearing around the house or sitting down (if you want to sit down and relax at the playground while your older one plays, etc., they tend to dig into you a bit more and be less flexible), but the benefit is that they will last you longer. I can still wear my nearly 3 year old in our Ergobaby carrier, so for the added expense, they are a good investment if you prefer them. They're ideal if you're looking for something that you can use longer term (6+ months) and for mostly outdoor/standing/walking things, like doing grocery shopping, going for a walk, etc.
A stretchy wrap will only be able to support your babies' weight up to 4-6 months (at this point we moved up to a woven wrap, which again I can still wear by nearly 3 year old in). But they are probably half the cost of a carrier and more flexible and comfortable. If you did decide to go for a Moby wrap and decided at 6 months, when your LO was getting too heavy for it, that you'd like to get a sturdier wrap like a woven wrap, the cost of both wraps together was about the same as the Ergo by itself, so it works out to be the same. Wraps have a bit of a learning curve, so you might feel like they are hard to use at first, but you'll get more confident. I definitely think a wrap is easier than a carrier now, but it took some getting used to. Really you can't go wrong though as long as you steer away from the Baby Bjorns or similar. The rest is just up to personal preference.
