get both !
the differences between the two cartridges can be significant or insignificant depending on what what you are ventilating.
the 30-06 has by far, and undesputably, the widest range of loads available, of almost any cartridge. the 308 uses a subset of these loads. if your sole purpose is to hunt game larger than squirrel in north america, it will not matter which cartridge you pick.
if you want to shoot high caliber accuracy games, use the 308, it has far superior internal ballistics over the 30-06. better, and repeatable interior ballistics translates into smaller groups on the paper.
if you are into efficiency, the 308 is a smaller cartridge that will deliver 30-06 velocities with less powder, however you will limited to a smaller range of bullet weights.
if you are a dyed-in-the-wool cast bullet shooter, get the 30-06. you have more choices for bullet weight, style, powders, etc.
both cartridges were military rounds that found their way into sporting arms. there is 101 years of history for the 30-06 and 4 or so decades for the .308. both are really fine rounds.
if you asked me what would i buy, i would get the 30-06. there is nothing you can't do with the 30-06 that you can do with the 308 !!!
sg