its still the torque the matters, it would be comparing the 2 different engines with all else being the same. ie same gear ratios so the torque would the multiplied by the same ratio

and its the torque at the diff thats important anyway. so if you had an engine with no torque but a huge reduction ratio then it would possibly need a similar strength diff to an engine with more torque and less of a reduction ratio

horsepower doesnt matter as the diff shouldnt really care what speed its doing when the torque is applied