Well, I didn't vote in the poll because I've only played a handful of games on either, so I'm not sure I'm qualified.
I have to say, I'm surprised that theme has come up so often in this thread. I have to admit I'm much more a fan of Houdini as a theme over the cellphone-centric theme in DI. To counter-weight that, I am a fan of Pat Lawlor games. In the end, I figured most pinheads would ignore theme and go with whichever game they found most fun to play. I find that a theme can get me to play a couple of games on a machine or avoid maybe it. Once I start *playing* the game, the focus switches to how much I enjoy the game itself.
I'm a big Star Wars fan, but never bought a Star Wars pin because I never enjoyed them (haven't played the latest Stern version though). I like AC/DC, but didn't get into the game (maybe because I couldn't hear the music that well). Never liked the carnival theme on Funhouse, but it ended up being one of my favorite pins. In the end for me: gameplay and collection variety might make a bigger difference than theme (although the whole package would be best).
I want to like Houdini based on theme, but so far I haven't found the gameplay that interesting. I'll keep playing it on location to see if it grows on me. I really like DI so far, but 66% of my collection Lawlor pins already (TZ, FH) -- so it might not be the best choice. In the end, everyone probably has different weights for theme vs. gameplay. I play lots with my family and friends, so we have a couple of lighter games (FH,MM), and just one deeper game (TZ) for me and my pinhead friends. My next pin is likely to be a deeper game with lots of flow. As much as I'd like to have a Houdini themed pin, it doesn't seem to fit the bill.
I'm hoping a future Beatles pin (love the theme) would be a good addition; but it's likely to be simpler, more accessible game. Normally, I'd just sell FH to make room for it, but I might have to pass since my wife loves FH.