Animals are at least an order of magnitude more expensive in pure energy inputs to successfully grow than veggies (this becomes apparent if you compare land usage between the two purely in solar wattage).
It's a complex equation because there are a lot of factors that have to come into play to do a fair comparison. It's probably as complex to model, and as sensitive to input-variable selection, as predicting global weather patterns.
Larger investments of energy for less food = higher energy cost for food. update: http://en.wikipedia.org/wiki/EROEI
Meat is only more efficient when you can't grow crops on that land, e.g. sheep and goats on mountains.