How to measure.. capacity to suffer or intelligence? Both are a active fields of research, the former in the neurobiology, animal welfare, ethology space.

I should add though, I think the science on the nature of sentience and suffering is not quite at a point to support some of the recent proclaimations made on the subject. There's enough there though that I reckon the precautionary principle is defensible though.

Also, to be fair to the people arguing for reducing suffering, doing so can be defended in a entirely rational ethical framework, it need not be an emotional argument. I'm just not sure within that framework how defensible it is to assume intelligence and potential capacity to suffer are correlated for many of the 'higher' animals we eat.