Ask yourself what I'm going to use this gun for in the future? If africa's DG are on deck the 375 or 416 rigby would be a logical choice. However if no desire to hunt such animals(don't need +.375 to hunt african plains game just DG's), then a .338 might serve you better. Loaded with a premium bullet either will serve you well. Something to consider is shootability, the .338 and .375 aren't *****cats but they certainly haven't got my attention like a .416 rigby did. If your recoil timid in any way this is something you need to include in the desicion process and if possible try shooting ones of interest prior to buying is never a bad thing!
Good Luck