Re: Penetration.... 338 vs 375
As far as penetration in my experience-given identical shapes, construction, and terminal velocity, bullets of equal sectional density will give equal penetration.
A friends custom .375 Ruger just got recently finished. Test fired against steel the factory 300 grain Hornady went through the first target, and put a big dent in one behind it. It looked to be expanded to roughly 1.75". I saw the bullet but don't know the details of the steel. The guys shooting are experienced steel shooters and they were impressed.
All that said, bullet weight seems to add a bit of smash factor on large bone.
I've seen the Swift A-Frame .338 275 grain in action, and there is not much that walks, talks, squeaks, or squawks, I would not shoot with it.
Even the old Winchester 300 grain bullets penetrate great.
I've had great success with Accubonds, and while I have not shot the 300 grain from my .338 RUM I would expect it to at least maintain the reputation of 300 grain .338's for penetration, and maybe move the bar.
You've picked two strong choices, and I would choose based on an available rifle that was most suitable for the trip I had in mind.