I think the ballistics difference is over stated for shots inside 400-500 yards. A 180 TTSX at 3200 vs. a 168 TTSX at 2950 will be about 5" at 400 yds. For the OPs stated 300 yds either will be fine. Zero it at 200 or so, put the cross hairs on, add a little for longer stuff, and shoot.
I'd go 30-06, and if I ever needed anything bigger I'd get a 375.
