For decades, the factories used 26" as the barrel length for which to advertise the muzzle velocities of their ammo. We all know that few rifles come with a 26" barrel, and this confused people. With the advent of so many more handloaders, component manufacturers, and publishers of loading data books, barrel lengths are all over the place--anywhere from 22 to 26 inches.
Now that you know how to build a watch...
The '06, because its case is a bit over-capacity, does better with a longer barrel. The fall-off in velocity per inch of barrel length for a .308 is less than for an '06. Probably 75 ft/sec/inch for the '06, maybe 40 or 50 for the .308.
For open country hunting, I'd venture that 24" to 26" is best for an '06. You can get near-maximum performance from the .308 at around 22" to 24".
Now, lemme say that this is as I understand the relative performance, based on a bunch of reading over the years. I know a bunch more about what works with an '06 than I do with a .308...
If you're gonna hunt deer within 200 yards, it doesn't make a hoot whether it's 19" or 26". Bambi dies. The main difference is the noise. Shorter is definitely louder!
, Art