At point blank range, a 308 will be better. If you shoot the 140's from the 260 then at 600 yards wind drift is about half compared to a heavy 308 (175 SMK or eq). Beyond 600 yards, then the 140 is winning in wind drift and available bullet energy. If one was to make the comparison fo a light (150gr) hunting bullet in the 308 then the data would even be more skewed in favor of the 260 Rem.
So, at short range, either is fine. Past 300, wind drift on the 308 is going to make your life hard, even with heavy for caliber bullets. You will be glad to be shooting the 260 at that point, but be sure to be shooting 140's to get the full advantage. Go too light on the 260 or a crappy BC bullet and there might not be any benefit other than reduced recoil.
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