They're so similar that most shooters/hunters wouldn't notice any difference in performance. Like 111 said, the .30-06 holdstheedge when it comes to heavy bullet (>180 gr.), but that's about it. For loads using 180 gr. bulletsor less,you might only lose up to 100 fps shootinga .308 compared to the '06.

Remember that the .308 is essentially a short action .30-06, with the case cut down to eliminate the empty cartridge space not holding powder.