Depending on what you are looking for, both games offer something similar. I have both, and they are two that would be near the end of my 'for sale' list. Note that I'm also pretty able to step back and say what is good or bad about a machine outside of my like for that particular machine - so when I say things that aren't overly glow-y about the machines coming up, don't think I'm a hater, I just can see the flaws of a lot of machines I own and love.
BSD has a crappy theme for a pinball machine. It isn't family friendly and it's relatively ugly. A lot of the shots are too hidden for their own good, which turns off casual players. Why in the heck is there the chick on the dude's nipple on the playfield? On the flip side, Mist sure is a cool thing to watch no matter how many times you've seen it, and the way that it stacks things is really neat. Certain shots are near certain drains if you hit them.
IM has a great theme for a pinball machine. It is family friendly and I think has a really nice looking art package. It doesn't have anything as cool as Mist multiball on it (or, quite frankly, even close to Mist multiball) but the magnets do some awesome things with the balls. Any shot can be a safe shot, but if you are slightly off in your aim can also become a drain monster.
Both games are brutally difficult if you are playing them how they are intended. This does turn a lot of people off, which is a completely fair thing. If you are someone who likes to play a 20 minute game of pinball, neither game will satisfy you when you move in the posts to help you accomplish this. BSD will probably last slightly longer in your collection because of how the stacking works, but neither is deep enough in that sense. BSD earned like garbage on location, IM I have heard conflicting reports about how it has earned, both extremely well and not-so-well.
Cost wise, BSD is more than half of what IM is, unless you're getting a deal on BSD or screwed on IM. Since the VE zapped the value of IM as a game, I know people who have found them under $4k. I expect that trend to continue, as I know a lot of people that don't like IM at all because of how difficult it is. The same people tend to hate BSD for the same reason, but unlike IM the market isn't going to be flooded with a new release of BSD, so the price will remain the same or inflate slightly.
Having said that, to me, if I had to sell one, IM would be staying and BSD would be going. For me, I LOVE brutal pinball, and these both qualify, but the last thing that I said about IM is what gets me about it every time - EVERY shot that you take can be a drain if it doesn't work right. You need to have absolutely precision shooting to make it work right. When you drain, you drain because you screwed it up. I LOVE that about IM, and it's why it is usually the game that I call the best ever made.
If you like brutal games, you can't go wrong with either, but just like I did in real life, I'd buy an IM every day before getting a BSD.