I guess it all depends on where you live and/or where you hunt.
I'd have to say that from a practical viewpoint, the 303 would be a better caliber for most Americans. Over 90% of all the "big game kills" made in the USA are deer.
If those 2 were the only choices I was offered, I would go with the 450/400 because elk is the common game here, and we have LOTS of grizzlies too, so I would be better served with the larger caliber. I now have a 9.3X74R in a Ruger and I have to say, I can easily see why that caliber is so well liked in Europe and Africa.

Still.......... a 303 double in a light handy fast rifle would be a real gem.

Dang…………….it sucks not having a lot of money.