Originally Posted by
Sarcasm
How could you possibly draw that conclusion? H Heaven's Fall main hand, H Lungbreaker off hand is the best setup.
Off the top of my head, I'm thinking that switching from Lungbreaker/Rib Spreader to Lungbreaker/Lungbreaker would lower your Mutilate damage but increase your energy gain through Focused Attacks. Poison damage would remain the same since Instant Poison is normalized. I'm at work so I can't run some numbers, but intuitively I'm feeling as though Lungbreaker/Rib Spreader would win out.
As always, if you want to know what's better, use a spreadsheet.