Exactly, Tekken 6 was built for the PS3's hardware. Thus, the PS3 version will be the superior version. The reason why PS3 games are far more difficult to port to the 360 than vice-versa is because the 360's hardware specs are inferior, not to mention the media used for PS3 games versus 360 games (Blu-ray vs HD DVD). Blu-ray can store far more data than a HD DVD. They are going to have to compress quite a bit of things to get it to fit on an HD DVD if they truly took advantage of PS3's hardware.
I really wish more info was readily available about System 357, but I'll do my best without it.
1 - Arcade systems and console systems, even if they share the same hardware, cannot be directly compared like you are doing. How much RAM does System 357 have? Is it shared, like on the PS3? How many SPEs does it have? What's the clock speed of it's core? These things are important when saying that it's optimized for PS3.
Looking at what (little) info there is online, System 357 is not identical to a PS3. The PS3 cell has 1 PPE at 3.5 ghz, and 7 SPEs. System 357 has 1 PPE at 3.2 ghz, and only 6 SPEs, so it is not the same system at all. No mention of how much memory System 357 (or the Tekken 6 box) has, so can't compare there.
2 - The reason games might be difficult to port from PS3 to 360 is most likely not due to hardware differences but to software differences (I can't confirm that they are, as I have yet to hear of ANY game studio that develops for PS3 first, then ports to 360. Tekken 6 may be the first example of such a game. And no, Virtua Fighter 5 does not count, it used the Sega Lindbergh system).
The PS3 is extremely difficult to program for. This is why most companies develop for 360 first, because they can get the core game done faster. Then they can just port the system specific items. Overall a huge cost saving maneuver.
God it feels good to talk hardware, been too long. :)
EDIT:
I hope everyone's seen the
new Tekken 6 trailer. Awesomeness incarnate.
Awesomeness incarnate indeed. Having not yet even played T6 core, I'm excited to be able to try 4 new characters.