Go with the 30" barrel. I was told a 1.11 twist barrel is perfect for the 208grn A-Max and 210grn Berger. The 300 WSM cartridge is awesome. I have been reading and hearing that it's performing very well in F-Class and 1000 yard Bench Rest comps.
I just talked to a guy who just built a custom 6.5x300 WSM for Pdoggin past 1000 yards. He said the rifle is designed around a Nesika Model K Action and a Krieger 32", 1-8.5 twist barrel throated for the 139grn Lapua Scenar. He is pushing the 139grn Lapua Scenar at 3500+fps. He said it is unbelievably accurate.