To me, they are completely unrelated.
Sinister it's part of a program with a specific test, the Beast Tamer/Iron Maiden is just a strength test involving three lifts. Both share one thing that I always found very strange: they're not bodyweight dependant. Like
@Pavel Macek said, it's very difficult for not very big guys to press a 48 kg kettlebell. It's not impossibile, but because it's a specific weight and not something based on the individual, certain body types will have a harder time reaching it,
if they ever will. There was a Facebook post some time ago of an Asian guy that is the lightest Beast Tamer ever and he's small, but that's an exception: if you look up at the list, each man is pretty big or bulky (Fabio Zonin, for example, is shorter than me but he's got over 10 kg of lean muscle mass more, and he's kind of the tank build, one near the other the difference is very visible).
Technically speaking, I would take this two general assumptions:
- One can typically stabilize overhead more weight than he/she can press;
- One can typically deadlift more than he/she can squat;
Each of the two is to be considered under normal conditions (i.e. not a raw or belted deadlift VS an equiped squat).
Now let's see the different testing lifts in the two "events" you're comparing (all weights and implements equal - 48 kg kettlebell):
- Beast Tamer: one arm overhead press (press), loaded pistol squat (squat), loaded pull up (not contested in S&S Sinister standard).
- S&S Sinister standard: one arm swing (balistic hinge - similar to a deadlift to some extent), Turkish get up (where the hardest part is to me the stabilization of the weight overhead).
Although it is true that the S&S Sinister adds a time component, it sill lacks one lift (two versus three). Moreover, if can work up to the total volume of swings, chances are he/she will be able to also compress time; on the other hand, going up in weight with the pull up requires specific programing.
Also, to me the skill set required to perform a good pistol is very different (and harder to master) from those needed for a perfect swing or get up, but this is personal.
All things considered, I'll say that to me they're not even comparable and Beast Tamer/Iron Maiden challenge takes the crown of the hardest between the two by a good margin.
EDIT: I don't know anyone that has completed both, but I'm pretty sure that guys like Zonin and Oliver Quinn (both Beast Tamers) or Elisa Vinante (Iron Maiden) can comfortably hit the respective Sinister standards.