I've owned both and if I had the choice of just ONE big bore then the 458WM would probably be it. It is flat shooting enough when using a 350gr Hornady RN at 2500fps and the 500gr makes a good heavy bullet load. Bummer is that the 350gr and 500gr rarely shoot very close to each other. The 375H&H is much more versatile when ranges hit 200+ metres, and the recoil is significantly less, it's more flexible and the more I use the 375H&H the more I like it. Take your pick, they're both great!
Cheers...
Con