Does not matter who is more aggressive. If any cichlid tries to lock jaw with a piranha, said fish will be missing it's lips and probably a good part of its face. It's like locking a jaw with a set of razorblades...
In the wild, cichlids defend themselves against Ps by putting their back tail into the ground with their heads up, to avoid getting their fins being bitten off... Once their fins are gone, the fish would be rendered helpless and then would be a snack for the piranha. The cichlid won't try to lock jaws with the piranha, but instead try to protect itself from getting bit. Maybe throw some body bites to chase the piranha away...
Imagine seriously locking jaws with this:
View attachment 770312
A cichlid could go at it all day chasing around a piranha, and it will only take a single bite from the piranha to seriously injure and likely kill the cichlid.