I think it's insanely tough to make predictions on these types of matches when there is no history between them, or mutual opponents, etc. Both of these 2 are so insanely talented it's basically beyond comprehension.
All I can really think of is that I think Marcelo would have a slight advantage if the match were no-gi, but a lot of that is simply because there's less Rafa matches to look at no-gi. But...in his first two ADCCs, starting at only 19, the kid took out Leo Vieira, Robson Moura, Frazatto, and Justin Rader & Cobrinha twice each. Obviously with Marcelo there are dozens of matches to look at, and he has the best no-gi resume ever.
In the gi I think the match is closer than close, and a true toss up. Honestly I think it's pretty much a toss-up either way, and I'd love to see a superfight. Marcelo is my favorite competitor ever, and Rafa is one of my favorite competitors right now, so it would be such a cool match as a fan. I'd hate for either of them to have to lose, though.
Put it this way, I wouldn't put money on it.