I can't see that sentience and the quality of individual happiness are necessarily related. It may be that some other species is capable of sustaining a higher quality of happiness. If, for example, that species turns out to be a kind of duck, are John Stuart Mill utilitarians obligated to maximize the quantity of very happy ducks over happy humans?
I'm not familiar with Mill or Bentham. What would a "preponderance" of the evidence on both sides look like if there were to be a balance, I wonder? Can we even compare quantity and quality in any way?