Using a car carrier for a sleeping baby is less than optimal for extended periods of time. The carriers will set the baby into an inclined position, which can close up their airway by tucking their chin and letting their heads fall forward. We often see them lay in a "c" shape or have a side of rotational preference. This can eventually lead to flattening of the skull on one side as well as allow for muscles of the neck, back, and pelvis to become asymmetrical. Babies need to utilize carriers for the sole purpose of car safety and get ut of them as soon as possible to allow bilateral movements and stretching.
Wearing a baby chst to chest, is best for both mom and baby in terms of posture. It is giving the baby skin to skin time, it is easier to breast feed and more convenient for mom to have her hands free. The natural flexed "fetal position" is supported with baby wearing, even when worn on the back. It is important that babies are not worn facing outward, as that puts undue stress on their hips and curves their spine opposite of their natural curvature.