The big difference between the two diets is that subject A would be getting a high intake of saturated fatty acids, and subject B would be getting healthy un-saturated fatty acids.
What does this mean?
Well saturated fats are mainly found in processed foods, mainly meat produce, so fast food is FULL of them. Saturated fats are extremely un-healthy if consumed on a regular basis, they increase the 'bad' cholesterol, and can put stress on the heart.
Un-saturated fats are found in such things as nuts etc, also studies have shown that un-saturated fats can lower LDL Cholesterol, and raise HDL (good) cholesterol.
How will this effect working out?
Well in the long run if subject A and B had these diets, subject one would struggle in the weights and on the cardio, his heart would be under a huge amount of pressure and his/her work outs will suffer, where as subject B would have better work outs and not be putting so much stress on the heart. Also saturated fats turn to fat quicker and are harder to burn off.
When would junk food help?
If you are an ectomorph then fast food can be good for you gain those extra few pounds of weight, but fast food packs in a vast amount of bad fats and MODERATION would be the key....
Anyway im going off a little here.... going back to the original question would subject A gain fat quicker, if both subjects were of the same gender, age, metabolic rate, size, health etc etc etc, i would say yes due to the fat content and the carb content of the dough.