.375 is a much more versatile cartridge than .416 - which is really a DG cartridge - and as CTDolan says, the .375Ruger is the best thing to come along since the .338WM.
Having said that, I would say that a lot depends on what other calibres you have. If you don't already have a hard hitting 8-9mm class rifle (say .338-.366) then the .375 gets the nod; if you do already have one then go for the .416.
If I could have only one rifle and was actually going to hunt with it (i.e. as opposed to backing up) then the .375 Ruger would be at the top of the list. If it would also have to do duty as a back up rifle then I would go for the .416. The trade off would be that it is not nearly as good for longer range shooting , as is pretty much the norm in places like the Kalahari.
However the reality is that most of aren't restricted to one rifle, and I can't really see the point in travelling half way round the world with only one - after all, things do break. (I personally hunt mostly with a .338WM, but my guide/backup/DG gun is a .458WM. This is a good combo, which covers all normal hunting ranges.)
BTW: Note also that quite a number of people over here load their .375's with bullets heavier than 300gr - even as heavy as 400gr. This makes a big difference i.t.o. terminal effect on large game.
And, no, Ruger ammo isn't really hard to come by over here - at least not in South Africa. Its available at most big gun shops in the greater Jhb/Pta area. I am sure you will also find it at places like Polokwane. At worst you will need to "book' some ammo before coming. You can probably even buy it online and collect at the store.
Your call!