Ah this is something I've wondered as well. I have no idea how they compare as I've owned neither and don't think the online demos do them justice. They both sound fantastic to me. The Boomstar has more features. But an Oberheim is an Oberheim. The one thing I would say is if you want the SEM get the Oberheim, but if you are looking for a different take on the SEM, the Boomstar has it. I really want the Boomstar since buying the 4075. I also really want to play the Oberheim though, but kind of for the purpose of hearing in person what I would be missing if I didn't have it.
"It ain't the instrument, Baby!" - Ray Charles