I don't know if this is true or not, but I have heard that butterfly
koi are slower growers than standard koi and that they are less
destructive too. I can tell from my personal experience that they
might be smaller - my koi supplier mentioned that his butterfly tend to
be smaller than standard koi of the same age and my smallest koi is a
butterfly even though it was larger than most of the standard koi when
I bought them 2 years ago.

I don't know if they are less destructive or not. With mine I saw it
positioned in a nearly vertical position attacking a harmless pot so I
don't know if they really are less destructive. (The lily that lived
in it had already been knocked out by another koi) Maybe the extra
long fins intefere with the fish's trashing motions so they don't have
quite the same amount of destructive power on the plants - sortof a
"spirit is willing but the flesh is weak" kind of situation

I have also heard that butterfly koi are hardier than standard koi due
to the Indonesian carp genetics they contain. This didn't turn out to
be true in my case as my only fish to die since I bought them all 2
years ago was a butterfly.

I think they look much nicer than the standard koi and I wish I had
used all butterflies 2 years ago.