Sure...
The FS9 standard texture is saved as an bitmap in DXT 3 format. That's it...
FSX on the other hand... The "specifications" given in the SDK require/recommend
1. The standard texture is saved in dds format and in DXT 5 format
2. The bumpmaps are defined in separate texture sheets
3. The specular textures are also defined in separate texture sheets
So for a start, FSX uses a combination of three textures in dds format. You don't "need" bump or spec, but hey, the eye candy factor is what it's all about.
Do fS9 textures and models work in FSX? Yes. But an FS9 model cannot work with bump and specular. FS9 textures usually have shadow and reflection/shine painted on as a static effect. FSX adds dynamic shadows etc at render time.
Basically FS9 paints can simply lack the punch of an FSX paint but do work. Of course - good painters have made great FS9 paints that can and will fool you. And as usual... much depends on the developer for the quality of a model - you can get crappy FSx models as well. Surprisingly many - sad to say.
But once converted to FSX you may find there's no going back. I thought I'd use both, but I ditched FS9 within days and haven't looked back, despite having spent well into the four-figures on scenery, aircraft and extras on FS9 stuff.
Models that are sold as FSX and FS9 compatible simply means that you usually get one FS9 model that has been patched so that it will work in FSX too. In other words - it won't have the features of a true FSX model.
But once you have flown an A2A plane with accusim, you'll KNOW why FSX is so pulling.