I fly with a pet carrier on average once a year, always in First. My rule of thumb is that if it’s an Airbus aircraft I select an aisle seat and if it’s a Boeing aircraft I select a window seat. This tends to allow the most room for a pet carrier in my experience.

That said, I was on the A321neo about two weeks ago (without a pet) and noticed how both seats were extremely limited in underseat storage whereas the non-neo A321/320/319 seemed to have more. That said, I would still select an aisle seat on the neo.

Any specific Delta pet in cabin questions feel free to message me or post in this thread. Have learned lots over the years and always happy to pass along info.