I will put it into a non-biased review here, since I've seen a number of posts which sound like they're trying to put X10 down:
I've used both FPS Creators, and personally, I feel that X10 is the best for really understanding what your level will look like since the textures in the editor are not reduced, unlike X9's which makes it harder to judge the overall outcome. From out of the box, X9 has the worst graphics, simply because it works with Pixel Shader 1 and 2, using weak shaders which have little effect. X10 on the other hand comes with parallax mapping, bump mapping and tons of other stuff where some can even be modified in the test game.
However, once you come to mods, X9 simply has the advantage over X10 as there are almost hundreds of X9 mods which will improve the gameplay and graphics etc. but X10's Mystic Mod seems the best compiled mod I've seen, with X9's Project Blue and a few others coming after it. Sure MM's still glitchy and rusted, but it's because the X10's source code was released not long ago, and it's a little harder to get grips with than X9's.
The AI in both FPSC X9 and X10 is ok, but does have a few parts that definitely need a tweak which can be easily edited via the FPIs. People saying that you can't create new AI scripts are nutjobs, since, for example, opening a door is AI, picking up a gun is AI, anything which is coded for the engine to execute is AI: Artificial Intelligence. Once you get to grips with scripting, you can create a new character AI for your game, but stick to the current ones, be it even if they're that terrible. Both versions now incorporate Dark AI so there has been a significant change in the AI. On the other hand, X9's Dark AI still doesn't live up to X10's one, with X10 having the character options displaying built-in variables for the AI's behaviour, such as "Faction (Friend, Neutral, Enemy)" and "Can Hear?". It is much better than going to the FPI and changing it for every single one.
Then it comes down to compatibility. Both version can not run on many integrated Graphics Cards (my friend has a 1GB dedicated Intel GPU which doesn't work with FPSC), so there is no clear winner there. X10, however, needs much higher specification than X9 simply because of the
HARDWARE required to run DirectX 10. X9 is more toned down on HD detail and rests alongside Pixel Shaders 1, 2, Pre-3 (2.9) and 3.0. X10 works with only Pixels Shader 4.0, simply because X10 IS Pixel Shader 4.0. And, of course, X10 does not work on < XP, because it does not have the correct software and OS coding to support it. Hacks will install keyloggers and destroy the GPU's capability instead of improving it. X10 and 10.1 work with Windows Vista and 7, which are currently the standard Operating Systems now, so DirectX 10 is slowly becoming more and more popular in gaming and Art Design etc.
And the whole "X9 has more support" fanatic is completely biased too, X10 has a group of highly dedicated users willing to help.
A few of them are: SikaSina Games (me), The Next, le shorte and Bigsnake. Of course, there are much more

. We are basically the boffs of X10 and can help anyone with almost anything to do with X10.
Overall, both FPS Creators are spectacular Indie game development tools, but the decision is entirely up to you if you want super graphics or super performance and gameplay. So basically:
X10:
Gameplay: 6/10
Graphics: 8/10
AI: 7/10
Support: 7/10
Chance of redemption/revival: 8/10
Editor: 8/10
Overall: 44/60
X9:
Gameplay: 8/10
Graphics: 6/10
AI: 5/10
Support: 8/10
Chance of redemption/revival: 9/10
Editor: 7/10
Overall: 43/60
Of course, that was my outlook of the two engines, and as you can see, X10 won by 1 point, but both are basically as good as reach other, X10 is not a failure, I don't recall Lee saying he dropped X10, so it's a fluke.
Just my 2 penneth

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-SSG