I have been going through a similar dilemma (mentally). Currently I have a deposit on HS 30/06 and a deposit/hold on .375 H&H with a nice stock.
I will also purchase a .416...its my favourite caliber so this is a given. I will help finance this by selling off a few guns etc.
I am a sucker for nostalgia....so the dilemma is the 30/06 was at a fair price, BUT my heart says .275. I also have a .308 on a surgeon action, manners stock that takes AI mags etc. that I wouldn't sell because its like a culling rifle and is deadly accurate can take a bipod etc. The .375, well the timber is really nice, maybe impulsive.
The thing is I'll still want the .275, that's how I am, I'm embracing "Rigby" not really the caliber if you get what I mean so in the end I'd finish up spending more because I will want the Rigby offerings.
To that end I am seriously thinking about altering so the battery will be:
.275
.350
.416
Because I'm likely to still get those (.350 and .275) over the coming years if I follow through with the 30/06 and .375 and the idea is to trim down the collection.
I am wondering about modern handloads with the .350 Rigby and I am thinking if needs more than the .350 well bang you step up to .416 anyway.
Its all keeping me up at night because it is a lot of $$ I am going to spend and the mix needs to be right.
There is no doubt the 30/06 & .375 H&H are the more practical calibers.....and I still could go that way, the .375 I've go there really has nice timber.
So the OP think long and hard about what you want as its not chump change....in all of this the calibers are not all that different the .275 will kill just as well as .308 and vice versa, its about how you want to structure your collection moving forward and what you value, nostalgia/practicality etc.