It seems that Sherpa is the favorite- wish I had asked before buying, but I'm happy with what I got too, so it's all good
Just wanted to let you know know that my travels on Airtran went smoothly with the sturdibag size large- it fit fine under the seat and my new little boy was comfy and had plenty of room. The stewards and stewardesses were really nice and brought him water. I did keep him in his carrier during the flight and he just slept almost the whole way.