James hit the nail on the head, between all the long 30 cal mags there isnt much difference in ballistics out in the field. I have a 300 H&H, 308 Norma Mag, 300 Win Mag, 300 Wby Mag and a 30-378 Wby Imp Mag. Ballistics are simular over the spectrum.... of course you see diminishing returns on the 30-378...it just uses to much powder for what I would call little velocity gain over the others. But that's just me.
What does matter somewhat is that some my rifles shoot better and are handier to carry than others and that can be a contributing factor.
And like James said....if I have the need to throttle up I always have my 378 Wby and 416 Rigby to play with.
doubleA