I would go for the .260, but that isnt as common and ammo is a bit more difficult. Using that criteria, the .243 wins hands down. I dont feel that the 7mm-08 is any more common than the .260 when it comes to available ammo. All 3 are great cartridges. My son started with my .25-06... its a great caliber but will likely come in a bit longer package.
