Should I get a Sharpedo or a Crawdaunt?

I wanted Crawdaunt, but getting a Corphish with a good nature is turning out very difficult, seeing as you can’t grind them with the Dex Nav, only fish for it. And the chance of encountering a Corphish when you fish is a mere 5%.

I already have an Aron on my team, which I intend to keep and eventually evolve into an Aggron, so Sharpedo’s ability to mega evolve doesn’t factor into my decision.

Don’t bother offering to trade me a good Corphish. Whatever I use, I want to catch it myself.

So. Which of the two should I pursue?