30-06 is more versatile, you can go from 125gr to 220 gr (Squirrel to Buffalo) and it does better in normal barrel lengths (58-60cm) whereas 270 is limited between 130gr to 150gr and needs 65cm barrels to do really well. So if it shall be either one as a one for all, 30-06 makes the race by far for me. I have one.
From a hunting perspective this is all outdated, incorrect or irrelevent. For the record I favor the 30-06 simply because it is what I started with back in the 1970's and there is simply not enough difference with modern loadings to matter, certainly not enough to have both and I'm not selling my 30-06's.
There are a lot of the misconceptions today because the original 30-06 hunting loadings were almost exclusively either 180 or 220 gr bullets. The 30-06 was tradionally loaded very conservatively for many reasons. By the time the 270 was introduced gun and ammo makers felt more comfortable loading newer chamberings hotter and up to their true potential. During the 1930's the 270 with 130 gr bullets did offer flatter trajectory and with bullet technology of the 1930's the heavier 30-06 did offer an advantage on the largest game.
But a lot has changed in 80 years and most shooters are still repeating things that haven't been true in years. So what if the 30-06 is available in 200 gr+ bullets. They aren't needed anymore. With modern loadings and bullets a 150 gr bullet from either can be loaded to over 3000 fps, and do it from a 22" barrel. You can get 3100 fps from either from longer barrels. Both have almost exactly the same trajectory and both will kill any animal in North America equally well. In 2012 there is absolutely no reason to load a bullet heavier than 180 gr in a 30-06. There isn't an animal in the world that will tell the difference between .031" in bullet diameter. In fact the slightly longer .277 bullet will likely out penetrate the .308 bullet. A bullet gets penetration from sectional density more than bullet weight. A 160 gr 270 bullet will equal or outpenetrate a 180 gr 30-06 bullet if shot at the same speed. Which they can be.
In 2012 they are equals in every way for hunting. As a long range target rifle the 30-06 gets the nod only because there is more data, more research that has been done with that round and a shooter does not have to do as much R&D to get success. But with the right loads and right bullets I have no doubt a 270 could equal or even beat a 30-06 at extreme distance.