.app is pretty self-explanatory like .org, .xyz is more generic like .com. Using "hello" in either is great, I would place more value on the .xyz because it's a standard renewal so you can sit tight for the right buyer, with the .app it pairs nicely though a more limited pool of buyers but also carries 1k renewal you don't want to carry it year over year.
ps. dot-app domains won't resolve unless https is enforced.