The 30-06 is much more versatile than a 308
You can shoot up to 240 and 250 gr. bullets (for example, the 240 gr. Woodleigh) which have tremendous SD and penetration capabilities.
While the power difference may not be big with light bullets and/or short barrels (22" or less) with heavier slugs and/or long barrels (24" and up) the 30-06 pulls significantly ahead (especially if you reload or with premium ammo brands)
The -06 is truly the king of all around cartridges...from coyotes to polar bears you are well covered....if a hunter in North America would be limited to only one rifle, it should be a 30-06....