Lets look at little logic to this?
What does the original question mean? If we are comparing two cartridges we are usually suggesting one of 2 options, that being:
1. Is one cartridge generally more effective that the other for a particular purpose? or
2. Can I do one particular thing with one that I cannot do with the other?
Now for some insight:
The .25/06 has less recoil and blast than the .270.
The .25/06 has advantage when shooting lighter game/varmints/deer because of "shootability" and trajectory.
The .25/06 with appropriate bullets has taken too many elk/caribou sized animals to be called inadequate.
The .25/06 handloaded with a premium bullet has more than enough penetration for elk sized game if you place the shot as you should, and the ""shootability" factor can be an aid to that taking place.
Now for the .270:
The .270 has more recoil and blast than the .25/06.
The .270 is usually made into lighter more portable rifles than the .25/06.
The .270 has a heavier weight range of bullets than the .25/06
The .270 does not have the "shootability" the .25/06 does for lighter game or precise shot placement.
The .270 has a larger caliber and increased bullet weight range to "assist" an imperfect shot placement.
What does all this mean?
If you load like bullets in each, say TSX's in both cartridges, 115 grain in the .25/06 and 130 grain in the .270 and place the shots identically in 2 like sized and like positioned animals, you will find both cartridges have enough penetration.
You will also find any real difference will be based on the individual degree of alertness between the animals so lets assume several dozen are taken with each cartridge, now what will the difference be?
The real answer is "whatever the rifleman thinks will happen, will happen" If you think you will fail with one and succeed with the other, you will be correct.
My experience with the 2 cartridges is that there is no animal that can be taken with one that cannot be taken with the other, up to a point, and that is more determined by the range (distance) the animals are taken, as that can influence the impact energy in which the bullet strikes.
I personaly think that speed of penetration has more influence in killing efficiency than caliber, again, up to a point.
Those of you who have used the .257 Weatherby extensively will know that it definitely kills deer sized game more effectively than the .270. I have a friend that has taken eland with the .257 Weatherby and after several safaris, takes no other rifle to Africa these days loaded with 90 grain X bullets at 3700 fps.
Confidence in the rifle based on first hand experience will demonstrate any cartridges attibutes.
For myself. I like the .25/06 as it gives opportunity to have a .30/06 in the rack as well and if we play this game to its logical conclusion, there is no contest between the .270 and the .30/06 for the larger end of the game range.
The .270 is only the better choice if you stay away from the .25's and .30's and go heavier to the .338's or lower to the .24's or .224's on the light end.
Son in the end, your gun rack will tell you if there is a difference between the .25/06 and the .270.
