From what you have given of solely knowing propane vs. butane; propane would have more entropy because the smaller the molecule of gas, the more entropy it has.
If it was given in a rxn: 2AB + 5CD --> 3C4H10 + 2C3H8 , then you know that it is butane (C4H10) because it has the largest molar coefficient.
Is this the case or was the only thing stated in the question butane vs. propane??