To me your answer is very simple... You've got the RSM in .375 H&H and like it, so the logical twin to it should be the Lott in another RSM.

You can load it down a bit or just use Win Mags in it and you'll have a matched pair of thumpers. The .458" bullets are very plentiful and there are lots of choices. The Lott and Win are easy to load for also. If I'm not mistaken Rigby brass is a bit expensive isn't it?

I've been extremely impressed with my .375 Ruger RSM and IMHO it's a fantastic deal with all the features 90% of hunters ask for in a big game rifle. Just thoroughly test fire it for function, perhaps have it bedded and you're in business.