Everybody is excited about Tekken X Street Fighter and Street Fighter X Tekken (or, at least, I hope so) but do you think we'll be getting both versions or none at all?

Differences:
TXSF - Developed by Namco, uses Tekken 6 engine, cannot be worked on or influenced by Capcom in any way.

SFXT - Developed by Capcom, uses SSFIV engine, cannot be worked on or influenced be Namco in any way.

I would be surprised if it came out to the 3DS. It seems rather niche already, with 2 games getting developed to appeal to two small hardcore fanbases, so I don't see them expanding much beyond their original release.

I would definitely not call Tekken or Street Fighter fan bases small, or else they wouldn't be making a Crossover game (the second Namco x Capcom crossover to date) and they definitely wouldn't be making two completely different games with different art styles, gameplay mechanics, etc. not much has been said but Ono has confirmed this past week that more will be revealed at Captivate and both companies will meet up at E3 to reveal where they have progressed as of late. And with Ono excited to get more things onto the 3DS we may very well be seeing something happening.

People like Street Fighter. People like Tekken.

People who care enough about SF or Tekken enough to have a preference based on the engine and fighting feel? That's not that many people. If they released one title, it would have more appeal than this. With two separate games they're going to confuse the mass market consumers and interest will dip. As a result, sales for both will probably be disappointing unless taken together as one product.

Those are my thoughts anyhow.