Because of the muscle mass of Bison and the thick hide, I would go with a conical bullet. Now ... depending on what rifle you are shooting, I would adjust my conical to match that rifle. Since this is a traditional rifle lets just say sidelock. And many sidelocks are 1-48 twist. Or you would have mentioned different I am guessing.
With a 1-48 twist I would shoot a 460 grain Bull Shop Conical bullet with around 90 grains of powder. What ever is accurate in that area, then adjust your distance according to your accuracy.
With a 1-28 twist there are a number of conicals that might work real good. One would be the Thor Conical. All copper. And great penetration and expansion. Check with Sabotloader and see if Lehigh has came out with a conical for the Bloodline bullets. These all brass bullets should get excellent penetration and their expansion, while different then say a Thor.. if place in the right place should be very lethal.
The all lead conical again, the Bull Shop or No Excuses conical would again be a good choice. Here you'd want as large a charge that would be accurate. And that should be a good load.
No I am not saying a roundball would not kill a Bison. But you'd have to really have a stout load and your shot placement and distance would be very critical.
What rifle were you thinking of using?