I had really considered the Atlantic Blue. I ultimately decided against it because all the ones I saw in the store looked nothing like the vibrant blues that I'd see in pictures.
I'd have a hard time giving up the YT. I rescued a Sailfin and nursed it back to health, then she became a coral eating bully in my tank and she had to go, and it was still hard to get rid of her. She ended up in the owner of the LFS's 300G FOWLR at his home.
I'd say, if you see one in person that's nice, consider it, but don't order one online, because if they look like what I've seen, you'll be unhappy.
Also consider that your YT has had it worse, and gets along with other fish, some tangs can be cranky.
In all honesty, and I can't believe me, Mrs. Tang Police is saying this, if you really did it right, you could probably do both, although you might not want to get the Juvenile Atlantic Blue, since it will be yellow and slightly similar shape to the YT.
