The 26" barrel will get at most 30-40 more fps. Maybe even less. All rounds will get a little bit more speed from longer barrels, but some guns really NEED the extra length. Most magnums benefit from 24" or longer barrels. A 308 does not get a lot more speed from anything longer than about 22" and do quite well with barrels of 20" or less.
If you are going to be shooting at 1,000 yards that tiny bit of extra performance might come into play. At ranges of less than that you will never notice. All things being equal, a stiffer barrel is more accurate. Barrels of the same diameter are stiffer when shorter, so in theory at least the shorter barrel should be more accurate.
I'd buy the one I could get cheaper and have it cut to 21 or 23".