Mike,

If as you wrote this will maybe be your one and only buff hunt, I personally would rather use the 416 Rigby you have, and spend the extra money on either an extra buffalo trophy fee (or other additional trophy fees), and/or a few more days hunting/touring Africa. I think this would be of more memorable value in the long run than shooting 1 buff with a double rifle. Especially a rifle that you will be selling shortly after. JMHO.

Erik