Ahhh- typing at the same time as Postman, eh! He's right- differing opinions will show up in a discussion. Otherwise, there is no 'life' to the subject.

While both are very nice, powerful calibres, after shooting Curly's .450/400 with lighter paper patched bullets & smokeless powder in his Farquarson, I'd likely gravitate to the .450/400 instead of the .450 NE. I've had numerous .450's, .458's and never a .400. THAT's what I would do.
Seems to me, they both use the same basic brass, so brass is not a deciding factor - for me.

A further observation, is both of these ctgs. can use cast bullets in full power loads. In my 'bunch' of 84 sets of mould blocks, many of them are .458's, however, I do feel the need to have some .40's - another argument for the .450/400! YES!