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Neither will make concentric necks, in the beginning anyway.
When you reduce the diameter of a neck by any more than .003", you create runout by stressing the brass.
Reducing a .30 to a .25 will stress the brass and create alot of runout. When you fireform the case, the runout will go away. The remaining case, however, will retain all the flaws of the original. It is therefore important to turn the .30 cal neck prior to forming. Turning the neck will uniform the neck wall thickness. You then squeeze the neck down to .25 in several steps and the neck gets thicker. It does not get thicker uniformly, and you will have to turn again, albeit with less material to remove. When you fireform, you'll have a nice, uniform neck.
But why?
25-06 brass is cheap.
Case forming with used brass is a pain in the arsch because the brass is hard. You need new, soft, preferably Remington ('cause it's the softest) brass to work with. Otherwise you will curse the day you decided to get into the project.
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