Gosh, I've owned over 10 different carriers, with three children. I think if I had to do it all over again, I would go with a Baby Bjorn for 6 mos and under. They weigh so little at first, and the Bjorn was by far the most secure-feeling carrier I used for that age. Their necks are supported, they are all snuggled into your chest, and the carrier is very easy to use. It is also nice because dads don't seem to mind wearing it, which gives you a break! Also, you can tote your toddler along on your hip since the baby is in front. Slings just never felt secure enough for a newborn when you are actually running around chasing a toddler!
For over 6 months, I would do an Ergo carrier. This is a very versatile carrier that will really grow with your child. You can wear baby in front and on your hip. It also allows you to securely wear your baby on your back, kind of like a cloth backpack basically. It was the most secure feeling backpack I tried without getting into aluminum frame type of backpacks. At 6-9 mos, my little guys suddenly became great grabbers, and it was no longer feasible to keep them in front or even on my hip as I was trying to do things. So on the back they went and this was the best one I tried.
HTH!