For two-shoulder comfort and ease of use, I love mei tais! My dd spent a significant part of her babyhood sleeping in one on my back. If your child's arms are out when he falls asleep, it's quite easy to untie the top straps, pull them out and around to catch the arms/body/head inside the carrier, then retie. I found it much easier than a wrap for naps -- when dd was on my back in a wrap, if she fell asleep, I found it really difficult to deal with the head bobble or arms out problem without assistance from someone.
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