A few thoughts...
Caliber
The .375 is a great killer, but not a great stopper.
Because power at the receiving end is linked to power at the sending end, the .375 is the typical recommendation for a client's rifle because it has enough power for any DG that walks the earth, but it is still shootable by visiting sportsmen who may not have much high-recoil gunnery experience.
The .375 will kill any Buff there is, but the .40+ will hit them noticeably harder.
Bullet weight
Kevin "Doctari" Robertson goes at lengths discussing in his book
Africa's Most Dangerous the concept of momentum, in addition to the usual concepts of sectional density, Taylor's KO, and energy.
To make a long story short, his life work and experience as a veterinarian and PH supports the wisdom expressed in this thread by many: 300 gr is a much better choice than 250 or 270 gr for Buff.
However, the story does not stop there: 350 gr up guns the .375 significantly into .416 territory.
Unless the .375 is to perform one-rifle safari duty and also take PG - in which case the flatter trajectory of a 300 gr is appreciated - it makes a lot of sense to use 350 gr for Buff...
Hornady DG bullets
I remain confounded by the on-going confusion regarding the Hornady DG bullets offering (which tells me that a lot of folks who spill oceans of ink badmouthing the Hornady DG bullets have probably never fired one):
- DGS (Dangerous Game Solid) has been around forever and I am personally not aware of any issue. It is a classic lead jacketed solid.
- DGX (Dangerous Game eXpanding) has indeed had some issues with core separation early on.
- DGX Bonded (Dangerous Game eXpanding Bonded) has fixed the issue with the use with a bonded core starting in 2017.
I have no doubt that a fair amount of pre-bonded DGX ammo is still floating out there, and likely still not producing picture-perfect post-mortem bullets, but the old adage still applies:
at which point in the animal death did the bullet fail? A mangled bullet recovered from a dead Buff still killed the Buff, right?
For what it is worth, my own experience with DGX Bonded in .470 NE (the Hornady ammo regulates well in my Krieghoff) has been excellent...
.470 NE 500 gr DGX Bonded recovered from a one-shot kill Buff...
Use of solids on Buffalo
Considering that the modern expanding bullets of bonded variety (Woodleigh etc.), bonded partition (Swift A Frame etc.) or hollow point monometal (TSX etc.) are now virtually 100% reliable, the need to use solids for Buffalo is a thing of the past, especially on the first shot where the risk of hitting a second animal after a broadside pass-through in a herd is very real.
A few PH still recommend using solids on follow up shots up the rear end of a departing Buff, but I personally think that a TSX works even better for that purpose since it is but an expanding solid...
.458 Lott 500 gr TSX: an expanding solid... It will easily reach the North end of a Buff shot from the South end.
Classic Buffalo bullet choices
Anything to the contrary notwithstanding, personal likes and dislikes are important. By all means, folks should only shoot the rifles they like and the ammo who inspire them confidence. Confidence is a much underrated killing factor in the hunting fields...
This being said, it is the rare modern ammo offering that will not do the job, but some are clearly better than others.
I would use PPU or PMP or Remington Core-Lokt or Winchester Power-Point ammo in Africa if it was the only ammo available, and I have used with full satisfaction Nosler Partition in Africa, but I have a lot more confidence in A-Frame or TSX / TTSX (and their many clones).
As to double rifles, it comes down to what load the rifle regulates with, and what ammo is actually available. We owe a debt of gratitude to Steve Hornady for having resurrected the grand old British calibers. If it was not for him, few .450/400, .450, .470, .500 etc. would be in the field today and no-one would have followed his lead in loading commercially for them. Use the older non-bonded DGX for practice, and hunt confidently with the DGS and DGX Bonded if it is what shoots well in your double.