first things first: the whole "the lives of many out way the lives of the few" thing would be tossed around and for superman to contemplate.
but then you throw in the respective lives of both universes and it's pretty even in terms of loss of life.
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this would ultimately spark an alliance of both combatants (superman and picard representing the federation) pleading to both Q and mxy to stop the petty challenge.
long story short, picard and superman would bring up how boring it would be for those two if their favorite "victims" (picard and superman) are wiped out or rendered brain dead from the shock of losing everything they've ever know or held dear.
Q and mxy frown and look at each other in understanding, remark on how true the conclusion is and how they "though of it ending it anyway", then fix everything and teleport away.
nobody dies
PIS/CISless Superman could destroy the Federation one planet at a time, outmanuever and outrun their ships and destroy their fleets. He could hit all the important Federation worlds: Earth and Vulcan for starters. Or if he really wanted to be an ******* he could gain the aid of the Romulans or Cardassians.