HA!
The only real advantage I've found in the .257 is that "ordinary" 117-120 grain bullets tend to work very well, since it's almost impossible to push them fast enough to come apart on deer-sized game. They do provide a little edge in bone-busting over "ordinary" 100-105 grain bullets in the .243.
When using "premium" bullets their pretty much peas in a pod, though there might be a slight advantage to, say, 115 TSX's or 115 and 120-grain Nosler Partitions over anything available in 6mm. But it wouldn't make a heck of a lot of difference at least 99% of the time.
My wife deliberately shot a mature Montana whitetail buck through both shoulders and the spine at about 175 yards a few years ago with the .240 Weatherby and the 100-grain Partition factory load. The bullet went right on through everything.
Now, it probably wouldn't have on a cow elk, but then a 115-grain Partition might not have either. I did run into a guy from Bozeman who's wife put a 115 Partition into a mature cow elk. He told her to aim behind the shoulder, but somehow the bullet ended up hitting the shoulder bone, just above the big joint. It went through that shoulder, the vitals, and the other shoulder before stopping under the hide.
