They're both fantastic cartridges.
When the 257Rob is used in a modern rifle or even one of the K98 custom jobs, it can be loaded up to the same pressures as the 25-06 operates at. This makes them kissing cousins in reality.
Most manuals, list their loads at reduced pressures. Some manuals, specifically give higher pressure +P loads along with caveats to the hand loader to make sure the rifles they are going to be used in can handle a diet of the high pressure.
One other issue with the 257 Rob, is that many of the earlier commerdial models, although being strong enough were barreled with 1-14 twist barrels. They won't usually stabilize bullets over 100 grains. Some, will only stabilize bullets 95 grains and lighter. Even with a 1-10 twist, some barrels won't stabilize anything over 100 grains.
If you're planning on buying a late model commercial or building a custom 257 Rob, opt for a 1-8 1/2 twist. That will stabilize even the longest bullets available.
The 25-06, doesn't suffer from most of the above restraints. Other than many earlier rifles, have 1-14 twist rates again. The only later rifles that I have seen with such a slow twist, are now on custom rifles.
The difference between the 257R and the 25-06, in a late model rifle, isn't really enough to worry about, in practical hunting applications. That being said, if you're going to limit yourself to one 25cal rifle, go for the 25-06. It will do everything the 257R will do but just a little better on lighter bullets and really takes the lead with heavier/longer bullets.
Ammunition is also much more readily available for the 25-06 vs the 257R.
It's kind of like the difference between the 308Win and the 30-06. When you're using 150 grain bullets, ballistics are almost identical. When you start using the heavier/longer bullets, the 30-06 starts to shine.