I mean this sincerely and not as snark: if this is a question you have to ask, just use Nacl; don't design with ciphers yourself. Since there is a "right answer" to this question and a "wrong one", "convincing" doesn't seem like a good use of anyone's time.
The right way to learn about cryptography is to start by learning how to break it. If that's something you're willing to sink time into, try this thing we set up:
It's totally free and by the send of set 3, you'll have an appreciation for block sizes.