Either would be fine for Buffalo.In the Nosler reloading manual they include a few comments along with the loading data.The guy that wrote about the 45-70 told of his experience with it on an African Cape Buffalo hunt.He shot a mature male and the bullet punched clean through the big male and also penetrated through to the far shoulder of a female they didn't see standing along side of the male cleanly killing both.I would think the 450 would have produced the same results all else being equal.
Good luck on the hunt!