I would get the 308 and have it re-barreled to 260 or 243 at a later date or sooner.
I am shooting a 243 (actually a 6 Competition Match) with 115gn DTAC bullets and H-1000 powder. I pulled my last 6CM barrel around 3,800 rounds and it still had lots of life left in it. I just didnt want it to die in the middle of the season.
260s will go over 3,000 rounds and 300 Win Mags maybe 2,000 rounds. It all depends on how hard you run them.