Quote: "x10 has DarkAI vanilla"
True, but the implementation is by far not as flexible and mature as the X9 one.
Quote: "All models are compatible"
But none of the X9 Characters can be used within X10, unless they use completely different scripts and shaders - not to mention that TF341 and Viral Outbreak are X9 only and can't really be used within X10.
The way I see it is, that X10 slowly becomes the little brother of the X9 version, as it's more for the people that directly want to see results without getting their hands dirty (DarkAI Settings can be controlled in the Entity Settings easily, Ingame Gui to change the visuals), while X9 is the slightly harder to use version that has way more power under the hood - way more fpi commands, pixel perfect hit detection, now also supports post processing and water out of the box, entity shaders can be controlled via scripts ingame, etc.
Not to mention that X9 has way more Community Mods that constantly change it for the better.
-Doomster