The Mk8 had almost 300 kg of explosive warhead, 6 times more than the 12,5 inches guided torpedos quoted above, it blew apart the good old Belgrano with no mercy.
Torpedo's were are and always will be lethal against shipping. The warhead of the Spearfish was a little smaller than the mk 8, 750 lbs of torpex vs 800 lbs but not enough to make any difference, it was the reliability that made the difference.
Nope she had the Tigerfish as well as the Mk 8, but these in the early version were unreliable and a second version was on board but was untried, so they went with the safe option, the Mk 8