Calories could be a good measure - a whole chicken appears to be about 1200 and a cow between 500,000 and 600,000 - so a cow is about 500 chickens or so.
Calories is a great metric. Better than mass. However, what I really want to see is calories produced per a unit of time. It takes about 8 weeks to raise a meat chicken and about 80 for beef cattle. 1 Cow = 50 chickens.