Besides the faster growth, does Piraya possess a more outgoing personality?
Pygo tend to swim more and be more active than Serras, but can be a lot less aggressive. Getting a finger chaser or one that is aggressive is case by case for all piranha. You might get some that are aggressive and you might get some that hide all day. At least with a pygo they will swim.
My manny before I sold him used to hide behind a piece of decoration anytime I was in the room. I removed the decoration and he will still go hide away from me. The only time he was out was when I was not in the room.

