recs for carrier for 21 lb kitteh?

Definitely get the soft case. A cat that size will have trouble fitting in the hard cases that actually fit under the seats. Seats are smaller and more cramped than they used to be. With the soft case you can shove it under for take off and landing because it'll collapse a little (sorry kitty). Then during the flight you can pull it into your leg space behind your feet and give the cat a little more room.

When I flew 2 cats across country the check in people and flight attendants didn't even realize I had cats in the bags because they just look like a gym bag with mesh sides.

I have two International Airline Transportation Association approved carriers and can recommend them. These won't fit under a seat though, they're designed to protect the pet in the hold of the aircraft. I bought these in Australia before Blaise, Duchess and I moved over here. I think you'll find it easier to transport your cat in the hold, but you need recommendations from people who've used the airline for this previously. I would avoid any flight that requires a connection. When I transported my cats from Australia I used a pet transportation service (Jet Pets) who were excellent - they arranged absolutely everything so that was easy. I flew with Duchess domestically in Australia once and she travelled in the hold (no option as only service animals are allowed in the cabin) and Qantas were fantastic.