Bob,
While I have never personally used either the 250 gr Sierra or the 250 Partition Gold my gut is telling me that the Nosler would be the better choice.
A buddy of mine took a wild Woods Bison (~2000 pounds live weight!) a couple of years ago using the 250 Partition Gold shot from his 338 WM. The shots were from ~175 yards and both bullets were recovered on the off side after pushing through 3 feet of tough animal. LoL
Another accurate load in my 338-06 is the 200 grain Ballistic Tip over IMR 4350. Also don''t be scared to try IMR 4064 in this cartridge as well.
I have only tried Reloder 19 in a couple of loads but I found in my rifles I was getting excessive pressure signs before I reached the velocity I was trying for. Your results may be better.