I would recommend the SS (although the PP will work, it is designed to expand faster for thin skinned game so with that in mind your penetration will be about 10% less). The SS has plenty of expansion upon entry and I have recovered quite a few just under the skin on the off side. In an ideal situation with a perfectly broadside shot you could use either bullet but if presented with any type of angle at all I’d much prefer the SS. One buff gave me a very hard quartering away shot. The SS bullet entered behind the last rib of the left side and was found under the skin on the shoulder of the right side-bullet completely traversed the body cavity taking out both lungs. ( 500 gr SS, .458 caliber @2135 fps)
Just to give an apples to apples comparison , I’ve also shot buff with my 416 Rigby using a 400 gr SS (@2325 fps) with very similar results. Go with the SS and you’ll be very happy with the outcome.