Nothing stopping you from installing both at once. I have both installed. You sometimes run into issues with scenery conflicts but usually its just fiine.

But as for P3Dv4 vs FSX.
P3D4 is a 64bit program meaning no more memory issues.
P3D is also much more stable.
Dynamic lighting really adds a lot of immersion.  Airport lights actually light up the ground and any simobject it shines on including aircrafts.
Aircraft lights can reflect on the ground which I really like.
Also the majority of addon devs stop developing for FSX so its very rare to find a new addon.

I love  P3D4. Last I heard P3D5 has a bug with GSX where vehicle liveries don't work but that may have been fixed. I don't have v5. But you can have both installed at once if you have room.