If budget is not an issue and you have the required spec I would recommend getting X10 as the question here should be which is the better engine long term rather than how many people meet the minimum recommended spec right now.
1)X10 can do any theme as can X9 with custom content. Both come with sci-fi and ww2 out of the box (X10 has some exclusive content).
2)Using the latest X10 update all model packs can be used. Some require minor tweaking.
3)X9 for gameplay (using a mod), X10 for graphics.
Quote: "X9 has mods, support, and is faster."
X10 is faster. It uses a timer based system and GPU instancing which X9 does not have. Also X10 uses the full performance available with your GPU and RAM. There is also good support from the X10 users and the source code will be released once DBPro X10 is public.
Quote: "if you put the effort and time into making a game, you can make it look as good visually as an X10 game..."
X10 has fully working normal and specular texture mapping for segments which has not been made possible in X9 (even with mods) so that is not the case.
If you haven't already got the hardware for X10 then X9 will more than satisfy you with the various mods out there. If you have the hardware for X10 and want to get the most out of FPSC from a graphics, performance and AI perspective get X10.