It's impossible to answer, as there's some missing information. Clearly, a class A/B amp delivering 100 watts into some load is twice as powerful as a Class A amp delivering 50 watts into the SAME load. But you seem to be wondering what happens for other "difficult" loads. And that's where the only possible answer is "it depends"... on the impedance vs frequency of the load, the output impedance vs frequency of the two amps, the gain vs frequency of the amps, and the power spectral distribution of the signal. And if the load causes either amp to go nonlinear, then it really gets messy!
