Probably the 308... not a lot of good high BC bullets for the 25-06 and the few that do exist would require a barrel change for a faster twist than the Ruger will have from the factory. A lot more good bullets available for the 308 for long range work. A good 168 grain BTSP Sierra or the newer Nosler 168 gr. competition bullet would be great for long rang work, but if it is for practice for hunting, you should probably be using the same bullets you are planning on hunting with. Consider the 308 with the Nosler 165 Accubonds, or the excellent Barnes TTSX bullets if you can get them to shoot well in your rifle...
