Owned both. Owned BSD twice actually. Owned IM once, but another is on the radar.
Really tough call. Both are very similar in that they are brutal and fast if set up properly.
Both have their own unique gimmicks. Both make excellent use of magnets which I'm a big fan of. BSD is just fast as anything, but I think the magnets in IM introduce a huge amount of risk and random elements.
Personally I've been leaning towards some Sterns lately for gameplay. Also, personally, the IM theme makes the game slightly better.
But, that said, price is a factor too. A decent BSD can be had for a grand or more less than a nice IM.