totally different boards. The only real similarity is that they are made by the same company and they both have the word banana in the name. I'm not sure if that fooled you into thinking they are similar boards.
Figure out what type of riding you are looking to do and what type of board you want. Like the post before mentioned, the BM is for all mountain more aggressive riding and the SB is more park oriented and for more freestyle/playfull riding. Lib also has boards that sit between those two in terms of style of riding so don't just look at those two.