I highly recommend getting a .375 Holland & Holland Magnum (especially for a novice client hunter). I’ve been successfully using rifles in this caliber for hunting all the African dangerous game ever since my first safari to the Dark Continent back in 1974. For a very brief period (namely just one safari), I experimented with using a larger caliber (namely the .458 Winchester Magnum because it was the only caliber larger than .375 Holland & Holland Magnum back in those days for which commercially loaded ammunition was being manufactured). But I soon reverted back to the reliable old .375.
It's merits are as follows:
- Easy availability of factory loaded ammunition/reloading components
- Wide range of available bullet weights (although the 300Gr is the only one you need)
- Flat trajectory
- Extreme accuracy
- Immense penetration
- Low recoil
The only place where it falls short, is for body shots on bull elephant. If you plan on hunting just the occasional elephant, then .375 Holland & Holland Magnum is perfectly okay. If I were to hunt a lot of elephant, then I would opt for a .505 Gibbs. But any decent caliber above .450 bore (such as .458 Lott or .450 Rigby) would be perfectly adequate assuming that you are using strongly constructed (either monometal brass or lead cored with thick steel jackets) solid bullets (reasonably flattened at the point) of 500Gr or above weight at a velocity of above 2100 fps.
Herein, I must add that the .375 Holland & Holland Magnum is an excellent choice for brain shots on even the largest of bull elephant from ANY angle whatsoever (even frontal brain shots).
If I had to choose between the .458 Winchester Magnum and the .470 Nitro Express, then it would depend upon whether I was more comfortable with shooting a bolt action rifle (in which case, I would opt for the .458 Winchester Magnum) or a double rifle (in which case, I would opt for the .470 Nitro Express). Personally speaking, by this point in my life… I can shoot both with reasonable accuracy. But when I was a beginner, I was far less of an accurate shot with a double rifle than I was with a bolt action rifle.
On basic principle (assuming that both calibers are loaded with 500Gr bullets), the .458 Winchester Magnum has better sectional density than the .470 Nitro Express. While the .470 Nitro Express has better case capacity than the .458 Winchester Magnum (which permits cartridges to be loaded to 2150fps without creating compressed loads or unhealthy high pressures that are particularly counterproductive in the heat of African terrains such as the Zambezi valley or the Kalahari). But sectional density is no longer as serious a factor as it was back in the old days (due to the much stronger construction of modern bullets even if they are slightly light for caliber). And the .458 Winchester Magnum can now be employed with modern powders to easily accomplish around 2087-2100fps in fresh hand-loads (depending on the rifle’s throat and barrel length).
So whichever you pick, boils down to individual preference. I do however, strongly believe that a budget bolt action rifle is a far more reliable proposition than a budget double rifle. I personally do not think very highly of Sabatti double rifles, and would thus go for the Winchester Model 70 Safari Express.
A small suggestion: If you do decide to use a .458 Winchester Magnum with lighter-for-caliber bullets, then please don’t go below 450Gr. Otherwise penetration and punch will get compromised regardless of how modern the bullet design is.
Oh, by the way. Your choice of using a .30-06 Springfield caliber rifle for plains game is excellent. A true classic. This has been my all-time favorite African plains game caliber as well, ever since 1974 (although I do like the 7x57mm Mauser as well).