We used a Baby Bjorn for the first one for about 18mo as she liked to face forward and not doing so induced much rage. After 18mo she insists on walking or just sitting on my shoulders. Its a lot slower, but she sees a lot more of the interesting creepy-crawlies and 'texture' of where we are. Every flower must be sniffed.
Second is quite happy on Amy's back with an Ergo at first, now upgraded to a 'Kinder-carry' which is similar but with custom sizing on the straps and baby bag (Ergo strapping was too big for Amy and the bag area was getting a bit small).
We did try a frame carrier, with the REI house-brand being the lightest that fit well, and it works well for faster/longer hikes, but the clientele gets a bit rowdy and has easy access to my skull.
-e