I need more time on Houdini. Last time I played, it wasn't clicking with me all that well, but I saw tons of potential. I can't make a call between TNA and Houdini at this point.
Alien, however, I would steer clear from right now. I'd be happy to play one on location, but I am not a real big fan of the layout. There's lots to do and see, sure, but I haven't ever felt a strong draw to keep playing. Unless you're shooting perfectly (and even if you are sometimes), the layout can be frustrating and overly punishing. I like hard games, but this feels just a little too oddball for me.
TNA is pure pinball bliss. Of all the time I've put in on the three games, I'd take TNA for sure, but I can't say definitively until I get more time on Houdini.