Time magazine tells us that E = m c^2 is the 'equation of the century'.
(or was it for the millennium?)
I was brought up with the mass transformation but am willing to go with
'accepted' conventions.
It appears to me that
delta E = delta m c^2
might be a safe way to express things to a novice.
Nuclear reactions involve different masses and calculating the delta m for
Hiroshima and Nagasaki seems to be a popular exercise.