Screwtop

If you are hunting water buff I would think either of the two calibres you mention would work - ie the .375 H&H and the .338 Lapua Magnum.

I do not own a .338 but they appear to be very similar in power and if you use well constructed 250 gr SPs and FMJs in the .338 I think with good shot placement they would kill buff very well. Plus Woodleigh also makes 300 gr pills for the .338 as well.

My personal reasons to choose a .375 over a .338 are:
1. The .375 is legal for cape buff and other big five in most African countries when the .338 is not.

2. The .375 is probably the most widely available big bore cartridge so finding ammo should not be as hard. Also reloading components are easy to get.

3. The .375 H&H Magnum is THE classic cartridge

4. I can use .375 projectiles designed for lever actions (eg the 220 gr Taipan, Hornady etc) for feral game and they are much cheaper

Having said that I have long been considering getting a .338 Win Mag, 8x68S or a .318 Westley Richards. One day but probably not for a long time, as I want the rifle to be in some sort of take down configuration for ease of travel and that adds a lot of $$$. So the .338 Lapua Magnum is similar but just burning a lot more powder. I like the idea of being able to use projectiles in the 160 to 180 gr range for medium game, but also load up to 250 gr and even 300 gr where it is required. Very versatile calibre range.

For my personal use I see a 8 mm to .338 as replacing my .30-06 instead of the .375. Read some chapters of WDM Bell last week and noted he recommended a .318 and a .450 as the ideal battery so you never know.

Also when I purchased my .375 I was actually looking for a .404 or a .416 but saw a nice .375 Whitworth instead at half the price.

Good luck in your choice.