When two gases are held in balloons at the same pressure, volume, and temperature, the quantity that can differ between the two gases is the number of moles of gas present in each balloon.
According to the ideal gas law (PV=nRT), where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and T is temperature, if P, V, and T are the same for both balloons, then the number of moles (n) can vary based on the specific gas and its molecular weight.
Additionally, the density and molar mass of the gases can also differ depending on the types of gas being held in the balloons. However, the number of moles is the primary quantity that can change between the two gases under the same conditions.