This is just me, but to 600 yards I'd take one of the Big 7's each and every day over the 300Wby.
My reasons, I refused to have a gun over 8.5 pounds (sling, rounds, scope) and I refuse to use a brake.
Added to that the Big 7's will have roughly 10 % less recoil, they'll shoot roughly 10% flatter, they'll penetrate roughly 10% more and they'll drift 10% less.
All in all to me and for my uses I'll go Big 7 each and every time over the big 30's.
Lastly, shooting them from less than perfect field positions will also push me to the Big 7's as well.
I used to shoot a ton load of chucks each summer, only used my big game rifles. My Big 7's were infinately easier for me to stay on my game with. Again, I'm not using heavy guns and I won't use a brake.
Just my way and my thoughts...