While the B12 unit looks about the same as what you've got, it is a stiffer shock and has adjustable compression damping. The 2G unit is a little longer than the 1G and (the 2G) will raise the tail of a B4 around an inch or so. Pitter put my old B12 shock on his B4 and was pleased with the results.
The Busa shock is a little longer than the 2G B12 piece and will raise the tail ~2". The Busa shock is also rebuildable, whereas the B12 is not. Given how cheaply either one can be picked up off ebay of various forums, it's often cheaper to just get another one though.