I think the used inventory at any given time is more random than anything. About a year ago, I was looking for a pre64 model 70 in 308 win. I knew they made a lot of them, but took me MONTHS to find one reasonably priced. Right after i found one, i probably saw 10 more the next month that were reasonable.
Regarding your question, really depends what you're trying to do with it. 375 is more versatile but 458 has it's place too, albeit more limited. I have both and like both.
There is no shortage of comments on the initial problems with the 458 when it was first developed, and powder issues. Some folks never got over those issues, and it was ever fully embraced by the masses. Those prior powder issues is a non-issue at this point with modern powders. Those early issues did yield the creation of the 458 lott. Certainly a more versatile cartridge, and does allow you to shoot a 458 win if you prefer a little less shoulder punch.