...which is of course zero.
...which of course is zero.
Which one is preferred?
I don't think there's much difference between the two. It depends on which word you wish to stress more: which or zero. (Incidentally, I would set it off with commas, but that is a matter of taste.)
This saves all the emphasis for the final word zero:
This puts the emphasis on which:
There is no difference in meaning, only in dramatic effect. The effect is greater if the sentence is spoken; if it is written, the effect is vanishingly small.
I personally would prefer
if I wanted to stress is or zero.
to me sound more like
putting more stress on of course.
This might be quite subjective.