If the price between an Eagle and the Buffalo is the same, logic demands that you go for the Eagle!
You need to know that both coins have the exact same amount of gold in them, one troy ounce. The Eagle just has some copper added to the mix to make it scratch and wear resistant, which results in a heavier coin.
Then there's the fact that the Eagle is an official legal tender coin, with the guarantees of a government backing its qualities. Counterfeiting legal tender will land you in jail much quicker than doing it with some privately minted rounds.