When we first started flying with Toto, we chose the Sherpa although United didn't specify. That was several years ago and I am still using the Sherpa. Toto loves it and because airline seats aren't always the height they say, it is flexible enough make it fit and she is still comfy. I always put extra blankies, etc. in there for her and dress her warmly this time of year because it's
cold down there. She loves it because the top unzips and she can keep her little head out and watch everything while we are in the airport. She is trained to only enter/exit through the front zipper door and I can just leave the top unzipped and tell her to stay and she does!
Good luck!