I've considered these same two as well (the SEM Pro actually). The Boomstar has far more features than the Oberheim does. But I've listened to all of the Boomstar SEM demos multiple times and there is something there that I do not like. I can't quite put my finger on it, maybe it's the resonance or the distortion, but I find I respond to it like nails on a blackboard. What I'm talking about is all over the Mean SEM Ranch demo SE has on their soundcloud. The Oberheim SEM
demos I've heard though, I've liked all the sounds I've heard quite a bit. The thing that's held me back is just the single sinewave LFO, and I've heard that the Oberheim can't self oscillate.