I would suspect, were one to do a poll, that at,least 75% of the rental/loaner PG rifles available for clients from outfitters are chambered in .300 Win Mag. I would suspect 75% of them will be launching a 180 gr bullet, and the remainder one in 200 gr. It works.
I have used a .375 in recent years, not because it is better, but because my last four trips have also entailed buffalo. Also, when hunting a wilderness area, I prefer enough gun to deal with whatever fauna-based issues may arise.
However, neither of those reasons are really relevant on a typical PG hunt in South Africa or Namibia. There, a 30 caliber is an outstanding choice.
All that said, my own personal favorite pure PG caliber is the .338 WM. It hits with nearly the same sort of authority as a .375, and has the reach of a .300. I am sure I am at least partially influenced by having a lovely custom Mauser so chambered that I have used on two PG safaris in Namibia. I also cleanly took a leopard with it.