Do not shoot either gun until it has been vetted by a double smith. Spend the money and save your fingers. He will look at wall thickness and the general condition of the barrels and make sure it is on face.
Nitro and black both do the same thing. There is a myth that black powder is safer. The only thing that you need to do is keep th pressure down to safe levels. Metal can not tell which you are shooting only if they are too great pressure. Keep the loads to 5,000-6,000psi and you should be fine. Higher pressure loads are more likely to make th stock fail than the barrels. 75-100 years old wood can be fairly easy to crack. A basic 1100 fps load with one ounce of shot will take care of almost all your shooting needs and is very comfortable to shoot and easy on older guns.