Vegetarianism and counting lives

By way of update to my last post, speculating on the relevance of the number of animal lives lost to meat eating, I see that the Dalai Lama has been quoted on the same topic:

An Indian friend told me that his young daughter has been arguing with him that it is better to serve one cow to ten people than to serve chicken or other small animals, since more lives would be involved. In the Indian tradition, beef is always avoided, but I think there is some logic to her argument. Shrimp, for example, are very small. For one plate, many lives must be sacrificed. To me, this is not at all delicious. I find it really awful, and I think it is better to avoid these things. If your body needs meat, it may be better to eat bigger animals.

Come on:   why did he have to choose prawns as an example?   If you're going to start worrying about prawns' lives too much from an ethical point of view, you're well on the slippery slope to fretting about whether you accommodate cockroaches and termites in your house.  Or bacteria in that infection in your foot.

I never really did trust this Dalai Lama - who himself is not a strict vegetarian, I see.  (For health reasons, but I also have read that Tibetan Buddhism, due to the great difficulty of having access to vegetables there, is not a branch that has ever been hung up about avoiding meat.)

Anyway, it still seems to me to be a good question - is it better to eat one relatively smart cow that has had a pretty free range, happy life for a number of years, or (say) 30 not very bright chickens that have been raised in a shed for 7 weeks and never seen the light of day?    And what if raising the chickens produces less CO2 and greenhouse gases?