Ngannou's chin is very overlooked, it's one of the keys of his success so far. He was getting hit by some heavy shots from Stipe and just walked through it, barely getting tagged. Also took some bombs from french journeyman Cisse early on his career.
Rumble's power is astonishing at LHW, but idk about it at HW. He wasn't able to KO Arlovski, although he dropped him and broke his jaw.
Personally, i feel like Rumble's power won't be so special at HW, so i'd go with Ngannou by KO, but the fact that Johnson can wrestle makes it interesting too.
I'm pretty sure Rumble is under contract with UFC and can't compete for BFC, Rumble posted somewhere that he's not fighting for them, they're just doing business with Rumble as a representation person.