I'd pick Rumble. In his prime, his chin was pretty much beyond reproach. In his entire UFC career, he never got knocked out or even knocked down. He has underrated offensive wrestling and -- most of all -- he is pretty much the poster child for "touch of death in his hands".
Pereira is the superior technical striker, but it's not like he goes through all of his fights looking untouchable. He gets caught and occasionally hurt on the feet, even by lesser opponents. Rumble would be a serious threat to him in any given stand-up exchange given his own power. What makes this interesting, however, are the low kicks. Historically I don't recall anyone ever making concerted effort to go after Johnson's legs. If Alex wipes them out early, it could spell trouble for Anthony's ability to close distance, generate power, and shoot takedowns as the fight goes on.
That being said, I have more faith in Rumble's power, durability, and wrestling than I do in Poatan maintaining distance, keeping the fight standing, and landing enough calf kicks to swing the momentum in his favor. We saw the blueprint on how to beat Johnson: it was to weather the early storm, drag the fight out, drain his gas tank, and then seek the RNC. It wasn't standing with him and trying to outstrike him. I think the fact that Anthony would present the most credible wrestling & top control threat Alex had ever faced on top of the ability to shut his lights out at any given moment would make for a combination that is sufficient to stifle the Brazilian's standard gameplan.