Which one is more Aero? P3x or P5D and how much?
We don't have any direct comparison data and we're really getting down to splitting hairs between these bikes. So there is no definitive answer without taking one of each and setting them up the way you would race then testing.
The table below shows a 5hr rider at IMNZ across the last 8 editions of the race. Time difference shown in seconds. A negative number means that the 2nd bike is faster.
The comparisons are all based on Cervelo data, except for the last column which is from Tour Magazin.
Light and Heavy refer to nutrition configuration.
So if we were to try to draw conclusions:
P5D slightly slower than the P5, depending on conditions
P3X definitely slower than P5X when fully loaded, pretty close if you strip it down
P5X faster than P5 if carrying the Kitchen sink
P5 much faster than P5X if running it clean (note that the Tour testing uses pedalling legs with no torso, compared to Cervelo using a static full mannequin)
For interest, P3X light vs P5X Heavy
Showing once again that carrying less stuff is 2-3mins faster and you can't buy your way out of that problem.
As mentioned - no definitive answer to your question. It seems likely that in the stripped down scenario the P5D would have an advantage over the P3X but that would narrow considerably once you're carrying stuff.
Connect - Velogic Fit
Or - Speedtheory