Right now, GSX and GSX Pro share like 95% of their code, so most of the features we add to MSFS automatically appear in the FSX/P3D version as well. Same for all new handling/catering operators which are added constantly, they are added in all versions as the same time. Same for sounds and voices: MSFS and FSX/P3D versions are identical and run exactly the same code.
Sometimes the feature might not be exactly identical, like the recently added Remote Deicing, which in P3D doesn't have a deicing-specific VGDS, but other than that, the feature works exactly the same and was added for all simulator at the same time.
Some vehicles/models has been improved, but mostly because they use MSFS-specific features, so they might not go back easily to P3D, and even if they could, they wouldn't look the same anyway.
But in general, I don't see many difference, other than the obvious graphic improvements due to the MSFS engine.
Of course, if a future P3D version, possibly which an better graphic engine that might be capable to obtain similar results than the MSFS one, and assuming it would be well received by users, might possibly justify porting back all the new models to it, assuming it will be technically feasible.
