I've seen old posts and I've played both games....and many, many others considered to be "brutal". In my mind these two are at the top of the mountain. Sure there are honorable mentions like AC/DC, Shadow and STTNG but they don't reach the utter brutality of IM or BSD.
To me IM reigns supreme. I've played both and yes I played BSD with the lighting flippers, but IM just has a nastiness to it that seems unrivaled. Right when you have everything set up it punches you in the face and it's all over before you even know what happened.
That being said I would love for people who own both to go into their audits and compare the average game times. I think that will tell the real story.