Well, it seems that UFC and WSOF are friendly with one another. The reason for this is probably because WSOF poses no threat whatsoever to UFC. WSOF is not seen as competition, it's looked at as inferior because it is, but to be fair pretty much every else is too for the time being.
So, with that being said, I couldn't see them ever pitting their champion vs. a WSOF champ, promote it as such, because say in fact Marlon wins, how does that make the giant UFC which is the WWE of MMA look? not good.
So, they will sign him first - if they have interest in him and book him as one of their fighters.
there is no way they will do promotion vs. promotion type fights unless they have bought out said promotion or promotion is in their back pocket.
I think they learned a thing or two from Pride and watching the old Monday Night Wars DVD lol ...