I think venomsnake was onto something a little more than you're giving credit for. I would rephrase it a little:

Diversity and plentifulness of nutrition, social/sexual opportunity, and economic opportunity are strong attractions of cities rooted in our core biological needs.

That more general form combines the ideas that the two of you present (I think efficiency is significantly enough intertwined with economic opportunity that it makes sense to treat them as a single factor). Different people may emphasize differing core attractions as reason for wanting to be in a city, but I think all of these factors are core to what attracts significantly large groups of people to cities, and I think you lose a large part of the attractions of cities by leaving out any of these factors.

edit: oops, originally cited the wrong commentor, also fixed a typo