There's not much difference in scenery development, except that some features (which FSX normally doesn't allow anyway) we worked hard to do in FSX, like directional shadows that changes during the day in CYVR, are not needed in P3D anymore, because the graphic engine supports them already.
There are some minor difference in the behaviour exported models, and P3D allows to export objects with an higher complexity but, in our case, we prefer to sort out those difference at run time using some functions of the Couatl engine, so we don't have to distribute two different installers, and for the time being we don't foresee making use of the larger ( > 65K ) vertex buffers in P3D, for the same reason.
And, unil LM releases a 64 bit version of the program, exporting extremely high complexity objects is not very useful, because memory limitations in 32 bit will start to creep in.
There's an entirely new Simobjects API in P3D, that we would really like to have in FSX, a couple of years ago when we made GSX, because lots of things would have been much easier, but since is not backward compatible with FSX, using it in P3D only and keep using the previous methods for FSX would be even more complex so, we'll not use it right now.
We believe P3D will really start to replace FSX when it will switch to 64 bit, something they must do sooner or later, in order to present it as a platform for the professional market in the long term.
The smartest purchasing option is probably the Developer subscription. You pay a monthly subscription, and you can even choose to pay for just one month without any commitment. According to LM site, this option is open to anyone, even individuals without any previous experience. And, looking at the EULA, there's nothing there that requires you to *publish* anything so, technically speaking, even making something as simple as an AFCAD, since it's an officially sanctioned SDK procedure, already qualifies you as a developer, regardless if you release it or not. It's funny that, in all the drama about the controversial "Academic" license, almost nobody realized the license that, to the letter of the EULA, is 100% legal for everyone, is the Developer one.