Well rb20, I can see you haven't got the answer you are looking for. Thats mainly because its impossible to answer unless you consider EVERY modification, gear ratio, etc.

The extra 6.5% stroke of the A15 will give you approximately 6.5% extra torque throughout the rev range until you run into breathing restrictions with the head and induction. With standard ports and carbs, the A15 will provide more torque until say 4,000 rpm. But due to the extra capacity of the A15, it will become short on breath at a lower rpm than an A14 will. Hence, an A14 will rev harder. Since power is a measure of both torque and rpm, it can be predicted by the flow properties of the head and induction setup. You may have noticed, a lot of american Chev/ford/dodge aftermarket manufacturers rate their products in HP. Eg, you can buy a set of heads which are stated to flow 600HP, not 250cfm.

If you look at the specs of the stock motors, you will notice they are basically the same power rating. I'm pretty sure the A14 just pipped the A15. This is with the same induction system and head ports, valve sizes etc. I think the difference (although small) is due to a cam change. But thats just a guess. Any performance datsun needs a cam change, a lightened flywheel and tidy-up job done on the intake ports anyway!!