Meat is currently the most readily-available protein to people and it contains all of the essential amino acids that we require (this is what is meant when you hear something referred to as a 'whole protein' or a 'complete protein'). But it is not the most efficient source we could cultivate. Things like soy and quinoa would be much better for the land that we currently use to raise animals, and the risk of diseases and infections are much lower with non-animal sources.
Biochemically, protein is not the most efficient way to store energy. Fat is. And biochemically, plant protein is not a whole lot different than animal protein; they both contain the same amino acids, and we can derive the same products from them.
So with regards to the use of land and efficiency there, plant-based protein is actually more efficient.
If you want to talk about transport, both meat and veggies get transported around the country quite a lot. I'm not a logistics expert, so you'll have to do your own research and come to your own conclusions, but musing on it just now I think that probably meat is less efficient to ship and store than vegetables. Vegetables don't necessarily have to be refrigerated the whole way, and meats do. Vegetables can be frozen at the point of harvest, arresting the process of decay, while meat producers put "Fresh! Never frozen!" on their meats as a badge of honor. A handfull of meat is heavier than a handfull of vegetables, so a truckfull of meat will weigh more than a truckfull of veggies, and therefore take more gas to transport.
Overall, I believe that Will is probably right: plant-based proteins are a good deal more efficient than animal-based proteins, and therefore probably more beneficial in the long run to our overtaxed planet.