I think Sub really is the heavy single target damage spec. it just has a bad rep.
Sub still has the positional issues and stealth weirdness, so that might still turn people off. Assassination has the benefit of an execute, but I don't know how far that'd swing things.
And yes, Sub does use rupture as it increases all of your other damage by 16%. Hm, now that I think about it, that could be the difference; Sub is very steady with small bursts, and Ass is more sporadic.