ORIGINAL: Aught Six
The Model T analogy is flawed. Whereas vehicles have improved greatly over the past century, bolt-action rifles really have not. It's true that the ammo companies are doing some really neat things with new and improved cartridges. But if a .30-06 kills animals just as dead as always, why even bother looking for some new fad cartridge?
Actually my analogy is quite astute.
Model T & 2004 Toyota Tundra= Both get you to the store, the Toy just faster (more efficient).
30-06 & 300 Ultra Mag = Both shoot the same bullet, the UM just faster (more energy, flatter trajectory).
Don't misunderstand, the '06 will work just fine most of the time. The only difference is other rounds usually carry more energy and velocity (flatter shooting) allowing for longer shots and more "killing power" with the same bullet. For the most part this is all moot until you get out past 300 yards anyway so it really depends on the hunter and what their expectations are. Many pros and cons to both sides of the issue so there is no right answer in the end, it's really up to the person to make the decision based on all the info currently available.
For me, I want to have the most efficient weapon to give my game the quickest, most humane kill I can deliver.