I have both and both would work. Many buffalo have been taken with the 45-70 for many years. In an Encore in factory ammo you could shoot Buffalo Bore 350, 405, 430 or 500 grain. The first three have muzzle energy over 3400ft lbs. Don't shoot these in contenders. Several loads in the 300 win mag would work fine (200 grain Nosler Partitions come to mind). I thing determining what range you expect to shoot at would be key. I personally would choose 45-70 (just for nostalgia) out to 200 yds and the win mag beyond that. Sounds like fun.