From a practical perspective...
If you love your 9.3x62 and intend to keep it, there is no point getting a .375 H&H; they are essentially twins.
A .458 Win gets you into a different category: the stopper calibers. Do not believe those who pooh pooh the .458 Win based on circa 1960's horror stories of compressed loads of Olin ball powder cooking under the African sun and producing squib loads the bounced back on DG skin. Those issues, while occasionally true - there were reliably documented cases - have been resolved for 50 years. Today's .458 Win loads are just as reliable as anything else, and, truth be told, as good as the original .458 Lott loads. So, if you can only get one, get the .458
What I would suggest however, is that you think long and hard about the concept of identical ergonomics, and IF the CZ 550 fits you well (?), it would make a lot of sense to buy both .375 and .458 and to sell your 9.3x62. There are endless advantages to getting a medium and a heavy that share the same ergonomics.