Typically a small bore gives up more velocity as the barrel shortens than does a large one. This isn't surprising as the bigger bore has more volume per inch length than does a smaller one. Having said that, the difference between the .260 and the .308 is not great, the .264" bore has a volume of 4.08 cubic inches in a 20" length of barrel while the .308 bore has a volume of 4.8 cubic inches in a section of barrel the same length. While the .308 would be my choice, there is no reason why a 6.5mm, .277", or 7mm based on the same case couldn't do very well in the all-round role.